Now the guy who compiled this data did his best with only hard confirmed data about actual sales (not how many were printed, but how many were sold).
He admits that his numbers are very conservative - for example, his numbers for Neil Gaiman are based only upon his Sandman graphic novels, as no numbers were available for his novels and short story collection and childrens' books. And only US sales data was available for George RR Martin. So both of those authors are probably actually higher on the list than they appear.
Unfortunately, there was no data available for older authors such as Asimoe, Clarke, Heinlein, and Bradbury - and inution tells me that all four would probably be on the list, with decades of steady sales for classics, and with Bradbury's books in particular being often used in schools.
There was also very little data available for some of the very newest authors such as R. Scott Bakker.
Nevertheless, this can probably give you a rough idea of where some of your favorite authors might be on the list.
For example, just one of Gaimon's books, the YA Coraline has sold twice as many copies as all of Steven Erikson's books put together, though Gaimon himself does not know how many copies it has sold worldwide (as he says in the comments). Now that blew my mind, given how many more Erikson fans than Gaimon fans that I know.
