As a CreateSpace author I can see real time reports of book sales and watch the changes in sales rank that result. Amazon reserves the right to make occasional adjustments to the sales rank even when sales are not occurring, and they will occasionally do this. The sales rank will climb during any hour-long period where there is no sale, and it will drop to a lower number when a sale occurs.
Amazon decided recently to credit the self-publishing authors with sales after the book was printed. We were previously credited with the sale at the time of the sale. Amazon is able to print books for Print-on-Demand sales in usually just a few hours. As a customer I have been notified of shipment within three hours of placing an order for a book that was printed after I ordered it. This shift to reporting sales after printing has has an odd impact on my daily monitoring of sales. Many of the sales now get reported while I am sleeping! Now I am often finding that I can wake up in the morning and see sales that were credited to me during the night.
Amazon sales ranking is very dynamic for books that are not best sellers. While books in the top 100 will not show a lot of movement throughout the day, books ranked lower can see their ranking jump several hundred thousand places with just one sale. I guess it is hard to calculate the hourly sales figures on an item that does not sell every hour, or even every day. Amazon wants the number to reflect sales during the last hour, and is not so concerned with the sales rank of less popular titles, even if they are selling consistently over time. As a result, you may see large shifts in sales rank for books that are not best sellers.
There is another fun twist to sales ranks. As a CreateSpace affiliated author I can ask that my books be listed in specific "browse categories" on Amazon. The sales page for each book will show the ranking for the book in the chosen sales category any time that it is in the top 100 titles of that category. My three books on Stirling engine construction all happened to rank well enough to be in a top 100 list at the same time recently, so I took some screen shots of the sales rankings to memorialize the moment.
The first book that I released is called "Three LTD Stirling Engines You Can Build Without a Machine Shop." Here is what the sales rank looks like for that title (click for a larger view):
I was very excited to see that sales were good enough to be placed in the top 100 sales for Science Experiments and Projects and Mechanical Engineering!
The second book I released is "Quick and Easy Stirling Engine." Here is the sales rank for that title (click for a larger view):
The third title I released is called, "Eleven Stirling Engine Projects You Can Build." It was also recently listed in the top 100 for two different categories (click for a larger view):