Amazon's reviewing protocol has changed over time. Amazon has always let any customer write reviews, but now they label reviews from people who actually bought the book or other item with the words "Verified purchase."
They used to require a customer who clicked "Write a review" to actually write something in the review box. I used to occasionally get annoyed-sounding reviews like this one, written by "Sj" about The Sixth Discipline
If you trouble reading the text, it says: "Again I leave a five star review for a book that I really liked but in order to leave that review I have to leave this review which is it’s a book I really liked. It’s so stupid to have to do this review wording crap." So, while he (or she) was willing to assign a rating of stars, he was reluctant to put why he liked the book into words.
Clearly, Amazon heard him or her. They stopped requiring a written review and allower "reviewers" to simply assign a specified number of stars, without any supporting commentary In some ways, this is bad because reviews are much better than mere star ratings. However, I think people like Sj are far from rare. An author might well get a much higher number of ratings than actual reviews. Since my recent promotion of Alien Bonds, it has gone from 32 ratings/reviews to 53, but only six of the increase had actual reviews attached to the ratings (those six were all very nice reviews!) .
Reviews are great, but I will take ratings if that's all I can get.