Also, it is possible that his troops were retreating so slowly that his towers got YOUR army routing, and your army routed/dissolved before theirs got away... Last man standing is a winner, even if he is limping away!
That could also happen if there was a paralyzed or unconcious enemy unit that your units didn't kill, and before they could they retreated.
And there are few reports of a bug in which a lone enemy is left with negative hitpoints, and game dosn't understand it should have died already. It could be called Druss syndrome (just read Gemmel's Legend
).