Yes. But who is actually buying these top sellers? Is the top 20 actually really representative of regular readers (Top 10 Music charts or Cinema grossing for example doesn't reflect real tastes).
Many I've never heard of.
Iv'e read about 14 authors in the the top 20. About 5 you couldn't pay me to read another of their books.
I have bought some top sellers. JK Rowling (but not as yet any post HB book) and Terry Pratchett (almost all). I have some Clive Cussler. I've never bought Dan Brown or Stephen King and have no intention of reading either. My wife has a bunch of books I might read eventually.
Also do you mean SF or F or SF & F by genre? genre on its own without a qualifier might mean something today, but certainly it used to need a prefix word.
I read Romantic, Adventure, Spy, Tech thriller, Detective, Historic Fiction, Fantasy, SF (all kinds, even TV /Film spin offs), Kids books. Certain genres I prefer.
Obviously if more books in particular genres have particular gender or cultural weightings and are published, then percentage readers have may not reflect any reader bias at all.
If in fact the majority of published books in areas I like to read are in reality biased, then you'd expect my reading to to be similarly biased if I chose purely on merit.
Why should ANY other basis than merit be chosen?
I've no idea of the ethnic mix of my library. But if males are over represented in SF& F publishing or detective, then I must think the female authors I have are better.
Without PROPER statistics (and top 20 grossing of anything isn't a proper statistic) I can't see were this discussion can go other that opinionated hearsay.