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've played this on 3 platforms now. Finished on PS3, then PC, now on PS4Pro. Also played a fair whack of online as well on PS3. An incredible achievement at the time, and still feels like that to this day, only to be surpassed and made to look dated by RDR2.