Good ad agencies devote a lot of effort to finding the best sites, and indeed the best sections of sites, for each individual campaign. Reach - The more times each individual user sees a particular banner, the lower the click rate will be. This seems logical, and it is a very well-established fact. For this reason, the total number of impressions bought on a particular site should represent only a small fraction of the site's total traffic. Different sites have different average numbers of pages seen by each unique visitor.
Some banners are much better than others at generating clickthrough, often for unfathomable reasons.
For this reason, a campaign usually starts out with several different banners. After a week or two, the banners with the lowest click rates are pulled out of rotation. It's well known that the click rate for any particular banner goes down steadily the longer the banner is up. To maximize clickthrough, rotate banners often. So, the click rate for a banner, or for a campaign, is affected by various factors.