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 only used once, had no problems. Cut hair cleanly. So they have already paid for themselves. Even if they go blunt in a short time, can't fault them as they are so cheap, they are value for money.