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 have been using Advocate on my dog for a few years. It covers all of the worms, fleas etc and only needs to be applied once a month. I also found the price of the product from Amazon to be quite competitive. It was also delivered promptly.