This next paragraph is just my opinion: I believe what happens is that most books on amazon are selling less on Amazon than we think--between 1-20 a day. Given that there's somewhere in the vicinity of 8 million books to be found on amazon, and over 2 million different books making sales in any given week, there are going to be LOTS OF BOOKS that sell the same amount in any update window/24 hours/week/month. So I believe what happens is:

Say between 7pm and 9pm tonight my book sells 3 copies. But there's also a thousand other books that sell 3 copies. Another thousand that sell 4. Another thousand that sell 5... you see where I'm going with this. I think Amazon ranks all the books by number of sales since last update, then within each group of books that have sold the same amount in that update, it weights the number of sales in the prior 24 hours, then the number of sales maybe in the last 48 hours, then maybe in the last week, and so on, and so forth. In that way, it knows the difference between the book ranked 12,310 and the book ranked 12,309 is two sales one week ago. It also knows the difference between the different formats of the same book, etc, etc, etc. But this has just been my opinion.