[quote=@Rilla]
Then why would anyone higher actually accept the fight?
[/quote]

Because they can gain 100 points?

They're not going to lose anything, and it's supposed to be hard to stay on top.

Plus, if the leaderboard system ever becomes telling of skill level, it's a low risk low reward motive, someone significantly lower should be fairly easy to defeat, giving you an easy 100 points. 

Plus, we're looking at very extreme examples here, it's more likely to be a fight between say a 1000 and 1200, where the bonus is only 20 points, which is not so significant.