I know this is probably a little late, but I just had a thought.
When you download something using IE6 under Windows XP (The current majority platform) you get that little yellow bar at the top that calmly warns you that what you are doing could be attempted suicide for you machine.
When you click on the yellow bar to allow it to download the file, it reloads the whole page. The same when you allow pop-ups for a website.
This would result in double the views for a rotating banner ad, and probably double the chance of a click-through ipso facto ergo elk, double the revenue.
So if you go with the flow, you can get users to see double the ads if you use pop-ups, or download from a redirect, and blame XP SP2.
IE7 carries on the behavior.
FireFox does not behave the same way, so if my theory is correct we should see an inverse correlation between FireFox update and banner add revenue 🙂