As the GM, my job is to make is to make sure that when the Batman takes down the Joker, it's because he earned it, not because his name is on the cover.
Why does Batman win? It's a very interesting question, with a lot of different answers.
1) Because he's the hero. This can be further broken down into:
1a) Because he's the protagonist.
1b) Because he's morally superior to the Joker.
2) Because he's more capable than the Joker. This could also be broken down:
2a) His natural talents are superior.
2b) He's better trained.
2c) He has better resources and equipment. Ironically, Batman didn't 'earn' these at all - his vast wealth is inherited.
3) Superior preparation. This is closely related to 2b, arguably it's the same category.
4) Because he made the right decisions during the conflict.
5) Luck.
I think (1a) is the real answer. The two foes are actually rather mismatched. Batman is a billionaire, with ten years of training by the world's best martial artists and an arsenal of weapons at his disposal. The Joker is a skinny lunatic in a fright wig. Ofc the writers have to make the battle a close run thing every time for reasons of dramatic tension.
There are analogies for most of these in roleplaying games. Batman might win just because he's a player character. Presumably the GM fudges die rolls or uses the many other resources at his disposal, such as his discretion in setting DCs and the like, to ensure (or make very likely) Batman's win. Alternatively, the system itself might make the PCs stronger than the opposition. There is a crossover into (2) here. Are the PCs better just because they are PCs or is there another, or additional, explanation?
Moral superiority leading to victory is something that occurs rarely, if ever, in rpgs. An example might be Pendragon where certain personality traits, such as Chastity, grant a bonus for being a good Christian.
The PCs winning because they are simply better than the opposition is, imo, by far the most common form of victory in rpgs, including D&D. The adventurers just have better numbers than the orcs, or whatever they are fighting. There are various ways they might have achieved these superior numbers -
i) Granted by system or GM.
ii) Min/maxing. This is, imo, an example of player skill, though a different kind of skill than I think Korgoth is talking about.
iii) Lucky die rolls for stats.
iv) Cheating.
Superior preparation is analogous to adventure-level planning, mostly non-system stuff, though there could be some system manipulation here, or a player could use out-of-character knowledge, like taking a flask of acid to deal with trolls.
4e is very strong on (4) - the PCs winning because the players made the right decisions in combat. In 3e it's easier to win at the character build stage.
The obvious analogy to luck is the dice rolls the PCs make during combat, rolls to hit, damage and so forth.
