Kurestin The publisher sends out review copies of the book to various people, who sometimes then sell them back to bookstores once they're done reading/possible reviewing. Those bookstores will then sell the small number of copies they receive that way (probably just two or three) ahead of the on sale date, at a steep discount. If you found just one or two copies in a bookstore that buys used books, this might be the reason.
Zoe Because book stores and libraries often receive books in advance of their publication dates so they can have them on shelves the day they are released. They aren't supposed to put them out before the official date, but I suspect that not everyone who works there knows this. The books would have come with a Do Not Sell Before This Date warning, but if that got thrown out then they might not remember to wait to put them out.