Aim assist aims between the head and the body at the percentage you specify. If you go to Cheer Park and scope in on someone you'll see what it does. If you set it to 25%, it will be aiming at the head 25% of the time. This doesn't mean 25% of your shots will be headshots.

On top of that, if you full auto at all, there will be some recoil, this recoil can make some body shots, like M4 shots, into headshots.

If you want to get your headshot percentage down, turn off Aim Assist and play use ESP only a few rounds. Or only use Aim Assist when in a rough bit. It'll come down pretty fast unless you're good at headshots normally.