[Kekephee]

It probably has something to do with roll mechanics. At 250 there's probably x modifier to a damage/hit roll upon swing, and beyond it there is probably some threshold that gets crossed that makes a big difference in the calculation. An example that I'm making up is, perhaps at 250 the bonus to chance to hit a level 60 mob breaks even with their innate chance to evade bonus, which means you are no more or less likely to hit them. With extra points, the roll crosses a threshold that puts you more likely to hit than if neither of you had any modifiers at all. Only slightly, but still ahead.

Just a completely unsubstantiated theory based on me trying to come up with a sensible answer to a pretty good question. I'd be curious to find out what the answer actually is.