Amazon's "sort by rating" functionality is fundamentally broken. See http://www.evanmiller.org/how-not-to-sort-by-average-rating.... for an explanation on how to sort items by ratings in an intuitively good manner. What it does is balance the uncertainty that a low number of ratings implies and the proportion of good ratings.

See https://news.ycombinator.com/item?id=9855784 for an older comment thread on that article.