Mel (Reviews In A Pinch) A lot of book bloggers get copies of books to review early. It helps with hype and usually ratings and reviews come from that. Occasionally people just rate a book based on the author or their excitement level for its release.
This answer contains spoilers… (view spoiler)[Not just ratings but whole reviews on books unpublished. I am wondering if it's the book's editors, literary agents or beta readers because only they would have some idea of the final product (though not of the final product itself). I have noticed a couple such reviews and they're disturbing. (hide spoiler)]