To calculate the overall star rating and percentage breakdown by star, we don’t use a simple average. Instead, our system considers things like how recent a review is and if the reviewer bought the item on Amazon. It also analyses reviews to verify trustworthiness.
They are quite shallow on the shelf I'm using them on. However this is more on me for not thoroughly reading the product description. Overall they work as described though, a bit difficult to attach to my shelves though.