Compared with other book review publications or programs, that is a significantly high ratio of books sent to books reviewed. Some folk don't have a knack for writing reviews. Some people don't appreciate how much work is involved until they try to write a review. For others, becoming a reviewer was just a passing fancy that passed all to quickly. There is a traditional agreement between the publisher and the reviewer.
Reviewers have the right to accept or reject a submission on any grounds they deem sufficient.
I appreciate publishers who notify me of any typos they may spot in a review. Even with two eyes proofing and a spell checker checking, a correctly spelled wrong word still manages to get through now and then. The book subject was inexpertly handled by the author. Space or time limitations prevent its use. For example, I'm doing a poetry column, and I've got room for 10 books.