Amazon calculates a product’s star ratings using a machine learned model instead of a raw data average. The machine learned model takes into account factors including: the age of a review, helpfulness votes by customers and whether the reviews are from verified purchases.
Honestly the first book in this trilogy was interesting and seemed fresh. But the next two were just meh. If you are reading the books in preparation for the movie, stop after the first 1. These last two were entirely skipable.