Math doesn't count when you OBVIOUSLY shoot him square in the head
Ok. I will type this one more time.
His "head shot hit box" is
not always the same as his visual head.
Look at your sharpshooter perk head shot requirement counter prior and after you kill a mob. If it count up by one, you killed him by a shot in the head shot hit box, if it did not... well... then you missed the head shot hit box and it will count as a body shot. Killing a mob with a shot in their head shot hit box is often pretty visual (his head will explode).
For example shooting the scrake between the eyes does not count as a head shot, but shooting it in the lower part of his mouth will. Maybe you aim too high
Shooting a gorefast center in the head will not count as a head shot (and he will just wiggle around and keep comming). Shooting him in the lower right (your right when he face you) side of his head (could say between his head and his shoulder) will make his head pop. Always, No matter what weapon you use. One bullet.
Unlike most mobs (bloat, crawler, clots...) that have a head shot hit box closer to the throat, the Siren head shot hit box is located between her eyes and her forehead (think upper part of her head).
Where different mobs head shot hit box are and how you should aim with your different guns is something you learn by trial and error while you advance on your way towards rank 5 sharp shooter. I had the game for two weeks or so and is still only sharp shooter perk level 4, but i pretty much figured out where and how to aim with my sharpshooter guns to pop them heads.
Everything (except Scrake, Patriarch and Fleshpounder) die in ONE (1) shot to the head hit box using any weapon (even bullpup that deal less damage per hit than even 9mm or knife, giving it the potential to score most kills per mag of all weapons).
If the actual head shot does not straight out kill it (which most lower ranked mobs do when using sharpshooter guns with the damage component of sharpshooter perk) - then it will bleed out over five seconds.
*hate that the forum take hours and hours to update. whats up with that?*