fatigue

why are 'fatigued' marines being sent into battle before the same level 'fresh' marines?

In the example below, MARDIV 1 fought 5 battles while MARDIV's 2-9 just kicked back and drank tea:

Delta Alpha Centauri

Attacker opened fire!

Wave 1 - Primary forces (Weapons & Boarding)

XINLOI Vs. Davy Jones' Mystery

Weapons assault
Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Target dodged the incoming attack!

Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Target dodged the incoming attack!

Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Hit on target, doing 8250 damage!

Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Target dodged the incoming attack!

Davy Jones' Mystery maneuvers into fire range and attacks first.
Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Hit on target, doing 8700 damage!

Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Direct hit on target, doing 10800 damage!

Davy Jones' Mystery maneuvers into fire range and attacks first.
Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Excellent hit on target, doing 12000 damage!

Davy Jones' Mystery maneuvers into fire range and attacks first.
Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Target dodged the incoming attack!

Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Target dodged the incoming attack!

Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
Light hit on target, doing 6300 damage!

Davy Jones' Mystery maneuvers into fire range and attacks first.
Davy Jones' Mystery attacks with Laser Cannon MK VII using standard mode.
The attack missed the target completely!

Boarding assault
XINLOI sending Power Armor Marines Assault Unit MK C to find a boarding spot. (attack 1)
The boarding party found an area and sabotaged it, doing 1007534 damage!
Target armor penetrated!
EMERGENCY JUMP!
Davy Jones' Mystery has retreated from the battlefield due to heavy damage!
XINLOI captured a group of 20 enemy personnel during the fight.

XINLOI Vs. Neliel

Weapons assault
Neliel attacks with Laser Cannon MK VIII using standard mode.
Direct hit on target, doing 12210 damage!

Neliel maneuvers into fire range and attacks first.
Neliel attacks with Laser Cannon MK VIII using standard mode.
Excellent hit on target, doing 14850 damage!

Neliel attacks with Laser Cannon MK VIII using standard mode.
Excellent hit on target, doing 12705 damage!

Neliel attacks with Laser Cannon MK VIII using standard mode.
Direct hit on target, doing 10725 damage!

Neliel maneuvers into fire range and attacks first.
Neliel attacks with Laser Cannon MK VIII using standard mode.
Minor hit on target, doing 4455 damage!

Neliel maneuvers into fire range and attacks first.
Neliel attacks with Laser Cannon MK VIII using standard mode.
The attack missed the target completely!

Neliel attacks with Laser Cannon MK VIII using standard mode.
Excellent hit on target, doing 14850 damage!

Neliel attacks with Laser Cannon MK VIII using standard mode.
Critical hit on target, doing 18645 damage!

Neliel attacks with Laser Cannon MK VIII using standard mode.
Minor hit on target, doing 5115 damage!

Neliel maneuvers into fire range and attacks first.
Neliel attacks with Laser Cannon MK VIII using standard mode.
Target dodged the incoming attack!

Neliel maneuvers into fire range and attacks first.
Neliel attacks with Laser Cannon MK VIII using standard mode.
Target dodged the incoming attack!

Boarding assault
XINLOI sending 10% fatigued Power Armor Marines Assault Unit MK C to find a boarding spot.(attack 2)
The boarding party found an area and sabotaged it, doing 743396 damage!
Target shield is down!
EMERGENCY JUMP!
Neliel has retreated from the battlefield due to heavy damage!
XINLOI captured a group of 144 enemy personnel during the fight.

XINLOI Vs. Vision Posessor

Weapons assault
Vision Posessor maneuvers into fire range and attacks first.
Vision Posessor attacks with Railgun Battery MK XI using standard mode.
Excellent hit on target, doing 36120 damage!

Vision Posessor attacks with Railgun Battery MK XI using standard mode.
Direct hit on target, doing 27300 damage!

Vision Posessor maneuvers into fire range and attacks first.
Vision Posessor attacks with Railgun Battery MK XI using standard mode.
Direct hit on target, doing 26040 damage!

Boarding assault
XINLOI sending 20% fatigued Power Armor Marines Assault Unit MK C to find a boarding spot. (attack 3)
The boarding party found an area and sabotaged it, doing 798765 damage!
Target shield is down!
EMERGENCY JUMP!
Vision Posessor has retreated from the battlefield due to heavy damage!
XINLOI captured a group of 106 enemy personnel during the fight.

XINLOI Vs. Rahonisr

Weapons assault
Rahonisr attacks with Laser Cannon MK XXXI using standard mode.
Target dodged the incoming attack!

Rahonisr maneuvers into fire range and attacks first.
Rahonisr attacks with Laser Cannon MK XXXI using standard mode.
Minor hit on target, doing 15300 damage!

Rahonisr attacks with Laser Cannon MK XXXI using standard mode.
Critical hit on target, doing 61200 damage!

Boarding assault
XINLOI maneuvers into boarding range first.
XINLOI sending 30% fatigued Power Armor Marines Assault Unit MK C to find a boarding spot. (attack 4)
The boarding party found an area and sabotaged it, doing 552781 damage!
Target shield is down!
EMERGENCY JUMP!
Rahonisr has retreated from the battlefield due to heavy damage!
XINLOI captured a group of 71 enemy personnel during the fight.

Wave 2 - Primary forces (Weapons flyby)

Wave 3 - Protected forces (Weapons & Boarding)

XINLOI Vs. Hijacked ship

Weapons assault
Hijacked ship attacks with Laser Cannon MK V using standard mode.
Direct hit on target, doing 8400 damage!

Hijacked ship attacks with Laser Cannon MK V using standard mode.
Direct hit on target, doing 7920 damage!

Hijacked ship maneuvers into fire range and attacks first.
Hijacked ship attacks with Laser Cannon MK V using standard mode.
Direct hit on target, doing 7920 damage!

Hijacked ship maneuvers into fire range and attacks first.
Hijacked ship attacks with Laser Cannon MK V using standard mode.
Light hit on target, doing 4920 damage!

Hijacked ship attacks with Laser Cannon MK V using standard mode.
Minor hit on target, doing 3600 damage!

Hijacked ship maneuvers into fire range and attacks first.
Hijacked ship attacks with Laser Cannon MK V using standard mode.
Critical hit on target, doing 13440 damage!

Hijacked ship attacks with Laser Cannon MK V using standard mode.
Target dodged the incoming attack!

Hijacked ship attacks with Laser Cannon MK V using standard mode.
Target dodged the incoming attack!

Hijacked ship attacks with Laser Cannon MK V using standard mode.
Target dodged the incoming attack!

Hijacked ship attacks with Laser Cannon MK V using standard mode.
Light hit on target, doing 4440 damage!

Hijacked ship maneuvers into fire range and attacks first.
Hijacked ship attacks with Laser Cannon MK V using standard mode.
Minor hit on target, doing 3000 damage!

Boarding assault
XINLOI maneuvers into boarding range first.
XINLOI sending 40% fatigued Power Armor Marines Assault Unit MK C to find a boarding spot. (attack 5)
The boarding party found an area and sabotaged it, doing 435690 damage!
Target armor penetrated!
EMERGENCY JUMP!
Hijacked ship has retreated from the battlefield due to heavy damage!
XINLOI captured a group of 23 enemy personnel during the fight.

Objective: Make the ship named Hijacked ship enter emergency jump.

So, as you can see, MARDIV 1 attacked 5 times while fresh troops were waiting to be deployed.
Does this mean that MARDIV 1 has to fight to the death before MARDIV 2 can be dispatched (and then MARDIV 2 dies before MARDIV 3 comes into play, etc, etc, etc...?)?

(edit because - I wanted to explain that when I say MARDIV, I mean Marine Division)