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.
The concepts in this read were exceptional and you need to read the series in its entirety (as I did here) to get the benefit. What was disappointing for me was the ending that appeared contrived as though the author had run out of ideas and was rushing to finish off the series. Having said that I read it all and enjoyed most of it.