Jane Austen suffers from not having a large body of work. Since this is “most popular author” not “most popular book”, having a large output helps immensely.
Homer even worse, of course
D’oh!
Another thing about that list: IIUC, it’s supposed to be cumulative: total number of books sold all-time, as opposed to in a given year, which is what the OP asked.
If we are going by all-time total sales, I’d expect older works to have an advantage: Homer, Virgil, Euclid, Plato, Aristotle, Plutarch, Herodotus, Dante, and of course the authors of world’s primary religious texts.
Hm, I didn’t even notice that the OP stipulated “in a given year”. That’s going to hurt a lot of the big names, because nobody needs to go out and buy, say, a copy of Shakespeare, if they already have it. I know that in the Shakespeare class I took in college, I just brought in the copy I already had, rather than buying the one the professor officially listed and put in the bookstore. Given that, the answer just might be “whichever ones just had a movie”, as Thudlow Boink suggested.