They aren't straight rankings. They have a complex algorithm that has several weightings and probably include some velocity variables that affect placement.

There's been lots of speculation but Amazon rarely gives more than hints about how they calculate their best seller lists:

"We base rankings on all-time sales, as well as recent sales that are weighted more heavily than older sales, so that our lists are timely and aren't always dominated by all-time best-sellers like Harry Potter." - Amazon Spokeswoman

Source: http://www.msnbc.msn.com/id/32336521/ns/business-us_business...