This happens to me, every now and then. It's possible it's people screwing around with their connection for an advantage if you ask me, since it's pretty rare but I've had it happen a few times with the same people. It happens when I kill someone, and they are obviously counted as dead for quite a long period, yet they somehow shoot me back well after my shot. The server is registering them as dead, and as that's the case they can't be awarded the kill. Normal kill trades happen at the same time on the other hand.