I think it factors in the artists which are in the recommended artists for each of your top 20. I'm guessing that those were just the artists recommended the most by the most of your top 20 banRAB.
EDIT - Addendum:
My list has Widespread Panic on it, and I really don't even like WSP - I don't even have them in my library. They've just been recommended by my top banRAB, I believe.