the official adsense blog has this gem: An explanation of adrank: http://www.searchengineguide.com/searchbrief/senews/006778.html While bid price (the price advertisers are willing to pay for each click per thousand impressions) is obviously an important factor, we also calculate advertiser Quality Score to make sure the best performing ads will always appear on your site. Quality Score takes into account factors such as an ad's relevancy and click-through rate (CTR) (i.e. how likely users are to click on a given ad) because a higher CTR sometimes outweighs the benefits of a higher bid price. This ensures that your site always displays the ads representing the highest revenue potential for you.
Yeah, that way Google makes the most money possible, by optimizing the ads. What, you thought they were in this for the fun?
If you get a lot of low-paying ads, that means your site's theme doesn't have a lot of high paying ads to be delivered to you. Google optimizes their ads to show the high-paying ads first, always, regardless of site. That way, they optimize their earnings. They don't choose which site gets low paying ads and which site gets high paying ads. Remember that their primary goal is to make money, and they do that best by delivering as many high paying ads to as many sites as possible.