Very few review sites will ever give a book anything less than 3/5 of whatever they award. As far as I'm concerned that turns a 3star book into a 1star, and a 4star book into a 3star.
I reviewed for a site that had a policy of nothing less than 3 stars. I sent a few books back because of that.
There are all kinds of reasons review sites over rate books. I mean, if you regularly give a publisher's books low ratings they're going to stop sending you free books. If you, as a reviewer (and wannabe author), give an author 5 stars on a regular basis, maybe they'll promote your book when you finally get it published. Review site owners and reviewers can end up in contact with authors and even editors. Great for networking and getting to know names (and getting people to know yours). And reviewers can get to read books from new publishers without shelling out the cash to see if it's the kind of place they want to submit their work. (I'm of the opinion that most reviewers either are or want to be authors)
Besides that, authors will link to the review site when they crow on their blogs or websites about the fabulous review they got. Drives traffic.
Respectable review sites? The ones that will actually give crappy books 1 star.