Generally, there has to be a good reason for a book (or video game, or non-independent film home release) to come out on any other day—like releasing a book on the Fourth of July, Halloween, or Friday the Thirteenth. Stuff like that.
Audiobooks, on the other hand, are often released whenever they're finished if it's not a simultaneous publication. Even with Dresden, you'll see Audiobooks come out a day or two after the Tuesday release, or ten days later, or whatever. (For what it's worth, I'm pretty confident that Peace Talks will have a simultaneous release across both mediums, because the audiobook versions are simply too popular).
