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.
Our cat takes some time to adapt to it, but now does well when we take her out in the harness. For reference she’s at roughly 4kg, is very fluffy, and the harness manages to stay on fine. You have to keep a firm hand though, as it can still slip off if the cat really tries.