Lag my friend.....me being in the UK, I sometimes hit a body shot...doesn't even register - blood comes out but doesn't affect the player....damn lag. I've shot someone with all 4 bullets before and they've survived lol.
As for the head, last night I shot someone in the shoulder and it counted as a headshot lol, I checked back in theatre to look..and yup shoulder hit.
I've got used to this kind of inconsistency being from the UK, but I agree it is quite annoying...when I'm sat yelling at my tv that it was a head shot lol.