The most likely reason is some level of desync between the clients and server. This manifests in lot of ways within the game, but is most obvious when shooting because of the short timeframes involved. i.e. the fact that the arrow was even in his airspace got to his client later than some of the arrow's shots.
Edit: This is also a great example of why CIG is pushing for 30fps on the servers. When they overload and fps drops down, a significant amount of error develops between where your client thinks it is, where the other guy's client thinks it is, and where the servers (which are the authority) think both of you are. This is the main cause of rubber-banding, and items/targets jumping around.