Amazon gets their books from a wholesaler, who gets them from Bella Distribution. Generally, all books ship out on pallets to wholesalers and distributors 2-3 weeks prior to the release date, after they are released by the printer. The time it takes for those wholesalers to inventory, unpack, re-pick, pack and ship books out to their clients (booksellers and others) can take anywhere between 7-10 days (generally).
On the other hand, books are shipped to independent bookstores and online retailers, such as Bella Books and Starcrossed, about a week prior to release date. Because the shipment is direct, these sites receive the books quickly and usually have them in stock, ready to mail out, before Amazon or most of the other large retailers.