Yeah, Butcher is one of the authors I'm probably underselling a lot (Robin Hobb as well for that matter, one of her label-mates suggested 10 million might be closer) but I am limited by available data.
The figures include all books in all editions (paperback, hardback, tradeback, ebook, limited editions, omnibuses, reprints etc). Collected sets, like those complete box sets of A SONG OF ICE AND FIRE or NARNIA or whatever, usually only have one barcode so are counted as one sale, I believe.
I don't know how publishers count graphic novel adaptations, or even if they do.
Big update at the original link, taking us to 199 authors, including the rather large oversight of Edgar Rice Burroughs.
Updated at the OP link. Scott Lynch and Scott Bakker have moved up and there's a whole bunch of new additions (Tamora Pierce, Tom Lloyd etc).