The way damage is handled by Arma2, each instance of damage (ie: each bullet hit) is handled 5 times, so the DZAI damage handler script is fired 5 times for each time an AI unit is hit by a bullet. The first instance of damage applied is the "overall" damage, followed by "head" damage, then body, hands and legs.
If the damage from the "overall" damage alone is enough to outright kill the AI unit, the AI death script is run right there and then and then prevented from firing again for the other 4 damage types. That is why a highly damaging gunshot is not counted as a headshot.
Long story short - this will be fixed in an upcoming update by moving a bit of code from one place to another. I am not 100% certain it will be fixed but fingers are crossed.