If you are an author who publishes through KDP, you've probably wondered how you can at times have one of your books sell copies while its sales rank on Amazon doesn't change. Or, conversely, have no sales and the rank does change. Here is Amazon's explanation:
The Amazon Bestsellers Rank shows how items in our catalog are selling in relation to one another. Because the Bestsellers Rank is relative, it will change even when there are no sales of your book, or fail to change when there have been sales. If your book sells a copy and the rest of the books in the catalog happen to be selling more often than normal, your Bestsellers Rank may not change. A sales rank of "34" means your book is the 34th bestselling item in the Kindle Store at the moment. If your book is listed in browse categories, it may also be ranked within those categories. You can find your book’s browse categories in the "Look for Similar Items by Category" section at the bottom of your book’s sales page. Category rankings will only appear in the Product Details section of a book's sales page if the book is among the Top 100 selling books in its category. This feature cannot be manually updated or added to a book’s detail page. In the Kindle Store, the Bestsellers Rank is divided into Free and Paid lists. The calculation is based on Amazon sales and is updated hourly to reflect newer and historical sales of every item sold on our website, with recent sales being weighted more heavily.