When people lose they just quit the game, they don't surrender. So you don't win the game, and you have to spend upwards of an hour finishing off the rest of their stuff. I've won 4 games, and I have credit for only one of those, this doesn't work very well in regards to stat tracking unless I spend most of the game mopping up the remains of my enemies.
I think it should automatically declare you a winner if all enemy players drop from the game, I have no interest in fighting AI opponents in a multiplayer game.