OK, My information was put together like this:
They see how many copies of all the seasons of whatever show have sold all together, since they came out. and that detirmines the best seller, except for the fact that some shows have more season, so the numbers were divided to represent an avergage amount of DVD's sold per season per show, and simpsons was #1. I think Buffy was pretty close though, and since Season 3 hasn't been out that long, it might beat it out.
I believe that is the fairest way of determining the stats though. Sorry I cant find the site, I have no idea how I got there..