Well, I'm not listed in the TOP 20, still I sometimes review a story. Strangely I feel more urged to review a story if I'm displeased with it. That leads to problems, because I don't want to offend the writer (although I sometimes feel very much like that). Writing, in general, is a good thing and I'd like to encourage people to write. Sometimes I simply can't. A good story doesn't need a review, it's enough to vote. A badly written story needs a review to tell the author how to make it better or to keep him/her from continuing.
Another thing is, that I don't like to be online that long to be able to read a story and then review it. I like more to download them and read them "at home". So I'd have to go online again to review it and normally I don't do that.