Apologies after reading through all the posts so far so still don't really understand what these stats mean.
You say current sales, does that mean:
1) Sales within the last day, week, month? (in which case, recent titles would likely have a big advantage?)
2) Sales of that book for all time? (in which case, wouldn't the books from years ago have a massive advantage over new titles?)
Is the ranking a pure indicator of "book A sold X books in the timeframe, book B sold Y", or does it have other factors, such as price/profitability/reviews/etc.