21 Jul 10
16 Aug 11 10:15 pm
I believe Google serves a page by what ads are available. If you had a page that was optimized for chocolate icecream and there are two ads available the top spot will be taken by the chocolate icecream ad at a cost that is 1 cent more than the second bid. Other spots may be taken up by people who are targeting icecream and then food and then diet until all the spots are taken. You can see how Google views your site in general in webmaster tools.
If there was only one chocolate icecream ad (cpc1.10) available and the second spot was filled by the icecream ad (cpc .15) the chocolate icecream ad would be displayed in the top spot at .16 cents not the 1.15
If you allow third party ads preference or google remarketing the ads may not be related to your site at all.
I have one site that is making about $60/day in adsense and one thing I did that increased earnings (doubled) without an increase in traffic was change the style of ads to appear on the site to Googles top performers. I use the 336x280, 468x60 and 300x250. These ads are the most used by people advertising with graphic ads and get a higher pay per click. Also having three different adverts means you get the highest bidder for each group. If you have two groups the same and the first group has three ads then the fourth highest bidder appears at the top of the second group.
hope this helpful