It's a sort of all-inclusive We.
Writing a book is something that you do on your own. I wrote American Gods. Me me me.
Making a bestseller is not something you do on your own. It's something that happens because of a lot of work by an awful lot of people: at the publishers, the marketing and publicity people, the sales reps, the person who decided to have little flashing lights on the lightning bolt on the dumpbin; then, equally as important, there are the booksellers; there are the reviewers, the truck drivers and warehouse people -- and most importantly, at the end of the day, the people who look at the book and decide that it looks like something that might while away a plane trip or a couple of days at the beach, and pick it up and take it down to the counter. (And then there's the people who read the journal, and mention to their friends that there's a writer named Neil Gaiman out there that they might like...)
It's my book. It's not my bestseller.