Amazon calculates a product’s star ratings based on a machine learned model instead of a raw data average. The model takes into account factors including the age of a rating, whether the ratings are from verified purchasers, and factors that establish reviewer trustworthiness.
The link for Marcia William's "King Arthur and the Knights of the Round Table" needs to be corrected. It does not link you to the listed book but to an adaptation by C. Louise March. Very misleading. Almost purchased the Kindle edition for the wrong book!