Not sure of the particulars, but I would venture a guess that advertising rates are dependent on traffic to the site, sort of like rates = newspaper/magazine subscribers
rates = ratings (TV/Radio)

Depending on the frequency the advertiser wants their ad to appear online (a constant banner ad), one that gets put into rotation, if it is "static" of a popup etc. Might be similar to the length of the commercial, time of day, particular program, column width, prime space like inside front/inside back or back covers, along with a host of other variables.