I don't like that. If anything here smells like Digg's "power users", this is it: the more you can post, the more karma you gain, and up goes your ceiling.
That's the point of the 100 most recent. If they are power users and they can submit 80 posts a day, each of which gets at least 20 points, then they are obviously making a contribution. If they just have high Karma though, they won't necessarily be able to have a high limit. If they keep submitting crap in the hopes that a few will be upvoted, their average will go down.
Also, you could try using median rather than mean and that might produce better results.
