Client side hit detection would be my guess. The kill cam isn't reflecting what the opponent saw, which due to lag is different that what you saw. Most console games use CSHD, including COD. COD seems to do a little more lag adjusting, so bullets don't kill around corners and assassinations happen before you face your assassin. It also asks the shooter what she saw before showing the kill cam.
The net code is problematic in Crysis 2. It's like Wanted:The Video Game at times. Bullets go around corners and head shot you. Ive been killed by a shotgun aiming at an empty screen. My personal favorite was jumping on to a roof of a building and going into camo, only to find out I died before the jump via a stealth assassination.
What I want to know is why headshots are so normal on unsighted weapons? It's as if the aim assist treats the body like a bullet magnet. About one fifth to a fourth of my kills are head shots, I'm not even aiming for the head. On this thought, what's the point of active armor if an unsighted light machine gun can have it's first bullet head shot you after you hit the target with three solid bursts of the assault rifle that starts with "G".