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.
The author says in her book that she really likes milkshakes, and it shows. There are often different balances of ingredients even among similar subsets of varieties, so she has clearly put a lot of effort into determining what works best.