I remember that learning about ranking on Amazon back in 2000 I found out it is not a matter of simply number of books sold. Ranking has to do also with a upward or downward trend and acceleration of that trend. Also, thousands of books are published every day so the ranking of your book(s) is immediately affected by that fact alone and can gyrate wildly. Ranking is important for visibility, but until you get within a thousand or two, it's not going to do a lot for you. I'm not sure what the threshold is for the ranking to begin producing increased sales. When I published my first book it climbed to 254 in a week, but so what? The number of books sold was maybe a hundred. True, there were many fewer books back then on Amazon, but the principle remains the same. What really counts is how many books you sold and whether you are happy with the number and the the money they earn you or the amount of fame or recognition you get, if that's what you crave.
Exactly - It also relates to the sales during the hour when it was purchased and then sales later.
I do not think anyone has ever been able to fully figure out how Amazon sales ranks work. I have also seen it stuck, sometimes for weeks, and completely disappear as if no books were ever sold. There was a time (perhaps in late 2009 or early 2010) when they stopped ranking many categories.
Got a reply today that they needed more time to look into it. My revenge novel now has a rank of about 160,000 with three books sold and one returned. That makes no sense at all. It should be up in the 400,000 or so.