i suspect either: on his screen your head is slightly above the barricade or on his screen the barricade isn't there XD
I think its more likely he was able to hit the top of your head tbh, not just once or twice I have been behind cover that looks to be tall enough when crouching behind it but nope, your head do stick up a bit. With the lower angle he has its kinda hard to say for sure though.
The FPS camera view is centered around chest height yes. It is easy to see just aim straight ahead at a friendly soldier in the warpgate, it will look into the upper chest/neck area.
Couple factors here: 1. The bullets you see coming from other players are artificial. Your client renders them based on a guess just to give you something to see. The client does not account for physical objects when doing this. It's why you see shots coming through rails/stairs/corners at times. 2. Your player model will change depending on the direction you look. For a barrier if you must be looking below horizontal to remain fully concealed. If you aim above horizontal, your head will become exposed on the enemy screen even though you appear concealed on your own.