Why does a direct hit from an overcharged plasma pistol in Firefight sometimes strip away the first layer of an overshield and other times completely deplete both layers?
Difficulty (heroic if in MM)
Skulls that change things like damage tables, or give “major upgrade to enemies”
Next time, try to note what difficulties are there, what kinda enemy (class of elite) and what skulls were on. Many reasons out there.