It's very likely built up so that you won't get as much back percentually from the defense costs (armor decay, healing) as you get from the offense costs.
An example, using artificial numbers;
- Offense costs: 5 ped
(ammo, weapon decay, amp decay. Costs directly related to bringing down a certain mob)
- Defense costs: 0.25 ped
(healing, armor decay etc to stay alive against said mob)
Let's assume 95% return on the offense costs: 5*0.95 = 4.75 ped
Let's assume 50% return on the defense costs: 0.25*0.5 = 0.125 ped
Total cost: 5 + 0.25 = 5.25 ped
Total return: 4.75 + 0.125 = 4.875 ped
Return %: 92.86%
If you kill the mob without taking any hits there would only be offense cost and return in play (5 ped cost, 4.75 ped loot). Less total loot (4.75 vs 4.875) compared to when taking hits, yes, but a better return % (95 vs 92.86).