Amazon calculates a product’s star ratings based on a machine learned model instead of a raw data average. The model takes into account factors including the age of a rating, whether the ratings are from verified purchasers, and factors that establish reviewer trustworthiness.
To be honest, I'm still trying to get to grips with it. It heats up extremely quickly and seems robust. It's easy to use, just not sure about my results (down to me though). For the price it's a bargain