It was never a good mystery anyway. How can there even be a winner in such a fight? Who's keeping score? Who's timing the rounds? Who's enforcing the rules? If someone was knocked out then there would be an obvious winner, but are two close friends really going to try to knock each other out? No cut man, no emergency medical crew standing by; to go at each other hard would be foolish. It would have just been a friendly sparring session, and "winning" is a meaningless concept in that context.
You think too much about the logic... This is what we call "movie logic". These kind of absurd plot divices are all around most movies.