Are you asking just out of curiosity or because you think it actually matters?
“Best seller” is really hard to pin down. There is so much secrecy and manipulation of the numbers, and outright lies by many. I don’t understand why it is that way, but it is. And of course there are so many “best seller lists”, each with their own “formula”, that only the really major players end up on the majority of them.
The important thing to remember is that for authors it doesn’t really matter what’s on the list. Even a western could end up on the list if, like Harry Potter, it hits the sweet spot with readers.