Listening and reading what very niche experts talk about sometimes makes you go wow. Jenstar, in a thread at Search Engine Watch forums named More ad units = less money? discusses how adding more AdSense units (ads) on a page can reduce the price per click you will be making on that specific page.
Basically, the more ads shown the more chance a lower priced ad will show up. So if you do show many AdSense ads on a page, the strategy will be for you to show the highest priced ads in the most prime spot on your page. How is that done?
Well according to Jenstar who uses the "AdSense support documents" as a reference, she says that AdSense ads are ordered by "Ad Rank" (CPC X CTR) in the order the AdSense code is found within the source code of the page. So if you want your most expensive ads (assuming the CTR of the ad is equal to other ads) in the prime real estate of your page, you need to strategically use CSS positioning in your code to make that possible. Or you can get lucky and assume the first AdSense ad in the source code is the most likely ad your user clicks on.