I have noticed this before, that people blow the brains out of animals, but without it counting as a headshot. But since that was with animals, I figured maybe the stat doesn't count for them. But here it happens with a human too, and still the headshot counter is at 0. What is up with that?
(https://i.imgur.com/JZX4Tnv.jpg)
(http://i.imgur.com/XuUrCVFl.jpg) (https://imgur.com/XuUrCVF)
Edit: Looking over the screenshot again, "Enemies downed (mechanoids)" can be removed since it will always be at 0, because mechs can't be downed in the normal fashion. They always die on being downed.