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.
...we were very disappointed when a month late we were charged $59 for "Amazon Prime Membership" which we DID NOT sign up for! We felt like we had been conned and tricked. Our business card details had been kept and used for something totally not linked to our purchase. Had we not been cautious and checking our credit card statements, this money would be gone as "daytime robbery". We feel this is utterly wrong doing on Amazon's practice.