Mr. Harrison seems to use his own secret, magical formula, then throws the names against the wall to see where they stick
(He actually provided some insight into his audience estimates in a recent Washington Post article about the size of Rush's audience, and freely admits that it's hard to come up with accurate numbers due to the nature of radio ratings. The "Heavy Hundred" seems to combine that with his gut feelings and observations.)
The figuring out which host is where, in the top, in one thing...but there are hosts that do appear to be on the list solely because they buy advertising space in the magazine.
"Duke and the Doctor", for example, shows up every freaking year. Who or what are "Duke and the Doctor", and are they on even 10 affiliates? When's the last time you heard "Duke and the Doctor"? Heck, by that standard, Ian from "Free Talk Live", a semi-regular here on R-I, should be in the top 100! At least he has 40 or so actual affiliates, mostly taking his weekend show, and some signals that are in actual cities with population.