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.
I only got part of the way through J.K. Rowling's "biography" before giving up in total boredom. The constant interrupting pictures made the text so disjointed that I just couldn't get into it - and the writing style wasn't interesting enough for more than one read anyway, even had the book not been illustrated. A total waste of time.