I think part of the difference there is that you haven't pre-purchased GRRM's next book. Maybe you're good friends with your local bookshop, and they've agreed

I think part of the difference there is that you haven't pre-purchased GRRM's next book. Maybe you're good friends with your local bookshop, and they've agreed to put a copy aside for you when it comes out, but that deal hasn't made it up to him. To use Gaimen's terminology, GRRM may be his publisher's "bitch", but at this level of success they're probably pretty happy to have him. His contract may in fact say "when it's done" rather than "June 3, 2014, minimum 20,000 words, and someone the readers love must die".