Duplicate of BUG-000092285: This is by design.
Each item details page includes a rating. Ratings are based on a weighted average of five stars, with one star reflecting a poor item and five stars reflecting a great item. The rating value provides a weighted average that factors in the number of ratings an item has received as well as the average rating. Therefore, an item with two ratings of four stars each has a higher rating than an item with one rating of four stars. To rate an item, you must be logged in and can only rate items you don't own. You can only add one rating per item. While you can change your rating, the count remains the same.