Today, I found an article published by the San Francisco Chronicle that backs this up...
A second reason for Groupon's domination surrounds its catchy offerings. A lot of the ads which show up on Adsense aren't compelling or relevant enough to get clicked on. However, a lot of Internet users utilize the web to thrift shop — and the Groupon ads highlighting daily deals can be very enticing for the user. So, this intrigue for users helps Groupon to further boost its click rates on Adsense.
The article explains why Groupon is dominating the AdSense channel as of late. The above passage was just one of the reasons why we're seeing so many Groupon ads.
But it reinforces my point that one of the key factors in ad relevancy is the AdWords Quality Score. The more money an ad makes for Google, the more often it gets displayed.
In this case, Groupon ads are making so much money for Google, that they're displaying these ads more often through AdSense, and giving them higher priority.
So they way AdSense determines which ads to display on your website is not purely based on which ads have the higher bids. But more specifically, which ads get clicked on the most, and which ones are earning the most revenue overall. That's defined as "Quality Score".
AdSense looks at contextual relevancy of your page and then selects a group of ads that match. From there it sorts the ads by Quality Score and displays them from highest to lowest.
If you feel your AdSense earnings are too low (and who doesn't?) consider that AdWords may not have had enough time to build up a reliable Quality Score on the ads that match your content. You might notice that overtime, your AdSense CTR/CPC/CPM rates increase over a period of months, or even up to a year depending on your traffic level, as AdWords is able to collect enough performance data to determine Quality Scores.
The way us publishers can help speed up that process is to make the AdSense ads very visible, make sure that Google can crawl our pages (submit a sitemap), and do what you can to increase traffic into all of your pages.