#1 Books presell now on Amazon and so it could easily have sold more than any other book recently as a presold item. #2 Amazon can recommend in essence any book they want as the "best book of the month". If it is just a recommendation then its completely subjective. If it is because it is ranked #1 then it is selling before it is even released.
As Seth notes, books can be presold on Amazon. That means that a book can indeed be a "bestseller" before it even reaches any buyers if there are enough copies ordered before publication (and this is entirely possible). Also, thanks to the Kindle, many books are available on the Kindle version on the same day as the book is released. Some publications which list bestselling books now have regular nonfiction and fiction bestseller lists as well as a combined eBook and regular bestselling lists. With the advent of electronic books, it is arguably even more possible to have a book reach the bestseller list. If you check out the ebooks which are bestsellers on Amazon, you'll discover that some are the same titles found on other lists and others are different. But the two can be nearly alike for some rankings, depending on the popularity of various books.
With Kindle's instant delivery, it is now easier for readers to post their reviews of books as well. They get the book on the same day as it is released and don't have to wait for it to arrive a few days later.
I bought it thru Barnes and Noble about a month ago. It had to be ordered online and I was in the store when I did it with their help at the help desk. But I got it half price and it came by mail within 3 days. I was told just because a book has a release date that only some books actually release on that date. Highly expected books I guess.