If anyone knows how to interpret this, please comment. I think it must mean that, if no one else sells a book in my category, I get my rank back because I haven't had an Amazon sale since 2/22/13. I don't see why my rank would go up otherwise. I can't bump anyone if I didn't have a sale.
I have heard they have complex algorithms, but I don't see how freebies and cash sales on Smashwords would even get in Amazon's data.
If you don't know, look at your rank and tell me what you think.