It could be...

Say you and I are playing a game and I destroy something onscreen, or I shoot you. If my connection to the server is better than yours, you might be lagging. Depending on how big the lag is, you might not have even seen the shot. My avatar might have just shown up onscreen for a few seconds and then disappeared, or you could unload an entire clip on my avatar and not killed me. Basically because on my console, I've already killed you.

http://en.wikipedia.org/wiki/Lag_(online_gaming)

There's also something called lag switching, which is a bit more sinister. It introduces artificial lag by disrupting the communication between a console and its server. Hackers do this to gain the upper hand over other players. While the other players are experiencing lag, they can pull off those sneaky kills.

http://forums.gamespy.com/battlefield_2_modern_combat/b49116/5433064/p1/ <<<an old post.