Another big factor that contributes to the outcome of fictional fights is the element of surprise. A writer can give anyone a chance at victory against a far superior opponent with a big enough margin of "whaaa?" That is why the fights featured on this site involve two (or more) combatants that are fully aware that they are about to face off against a worthy adversary.
What does that mean? It means I'm not going to bother to tackle the Batman vs. Superman debate. Comic book writers can let Batman win in a somewhat believable fashion by forming a story around a bat-victory. In the comics, Superman and Batman are friends at best and teammates at worst. Either way, they trust each other. That said, Batman could possibly get close enough to use a kryptonite ring from his lead-lined utility belt against the man of steel. I still doubt that would be enough to pull off a win, but back to my point...
If a villain, or even a stranger, approached Superman with an evil plot in mind, things would unfold much differently. Supes would hear a racing heart beat and determine malicious intent, scan the stranger's body with x-ray vision and ...