It's very common, as Murka said, it's not always consistent though, and has sparked a lot of arguments.
For me it's about 80/20 with the mob dead before the animation completes most of the time. When I run my lowbie scout, I think it's funny the way the mob always turns around so my arrow hits him dead on the nose, I think it's impossible to hit a mob from behind. Only time I really see the animation complete before the kill is when I have minor lag spikes.
I also tend to think it's just the way Rom is set up, and there isn't much you can do about it. The locations and 'kills' are handled on the server, the animation and models are on your computer, so when you attack it sends the signal to the server and starts the animation, if you have low lag, the server gets the 'kill' signal back to your computer before the animation of the attack is done so your comp starts the death animation while the attack is still going.