Actually, I know a little: It's based on an algarithm that scores Twitter users according to their influence on the web. I'm on Twitter as MarkRHunter (and as StormChaserBook, but since that's used only for specific news about the novel it's not very busy), but I think it's safe to assume I'm not Mr. Popularity there. Apparently this is all a much bigger deal to some people than it is to me.
For comparison, here's Neil Gaiman's:
It's my assumption that, since his full time job is writing and so Gaiman doesn't have to hold down a day job or two like I do, that he has more time to make his way around the internet.