It seems the 's some update from people who have dived into the game's code the adaptive difficulty seems to work in a peculiar way, all Zombies actually have different health pools manually placed on them so some will take more damage than others regardless but it seems the AD decides if they tank shots think of this as the reverse of critical shots where they take less damage on top of the differing health pool.
Depending on how things go you could kill a Zombie with a higher health pool than others which would take a number of shots then the AD could kick in and the lower health pool Zombie would end up tanking a few more shots than usual.