AdSense Weights Ads Displayed with Search Keywords

We discovered this little AdSense behavior of weighting ads on search keywords while checking the SERPs for a web site that obviously had AdSense on it. For some reason we decided to click a few of the links to the web pages from the various search engines and noticed that 2-3 of the ads in a skyscraper were different when clicking from a search than by directly accessing the page or from navigating to the page via site navigation. Based on our intial observations it seemed that secondary keywords using the same landing page are displaying ads weighted for that particular audience based solely on the search keyword.

Curiosity may have killed the cat but AdSense paid for his funeral so we did some testing.

How the test was done:

1 - To study this phenomena we set up a test with 2 computers so that my personal search profile wouldn't impact the results on the 2nd control computer.

2 - We chose a series of keywords to test using Google Analytics to find some top ranking search keywords that landed on the site coming from Google, MSN and Yahoo.

3- Then we performed searches on a series of terms that all went to the same landing page with a single AdSense skyscraper displaying 5 ads, in this case the home page, and clicked on links to the home page via the search page. We followed up this with directly going to the page without using a search engine to see what ads were displayed. [Later we tried deeper links as well.]

4 - We annotated what ads were being displayed coming from search keywords vs. accessing the pages direct and correlated the results.

What we discovered was that certain types of ads were displayed based on the keyword such as search for "Junior Widgets" returned a portion of the ads (2-3) as teen oriented ads compared to a search on generic "Widgets" or "Senior Widgets" or direct page navigation and the results were very reproducable over and over.

Knowing that search keywords weight ad targetting should help webmasters use a combination of Google Analytics and the new Adwords Keywords Tool to determine which search keywords are impacting their revenue and via SEO efforts enhance the more positive paying keywords to maximize the revenues of landing pages.

UPDATE - It's in the Patent!
Someone pointed me to the AdSense patents and Jenstar confirmed that she's known AdSense does this so at least I'm not imaging things! Funny I've never heard this mentioned in more than a year since I've been using AdSense and reading the forums but it's posisble that prior to using Google Analytics it just wasn't that easy to see these things going on.

MORE UPDATES
After doing a bit more testing it seems that only Google, MSN and Yahoo search queries are being used by AdSense to enhance the ads displayed. The queries of other search engines like Teoma, Gigablast and Altavista appear to be ignored by AdSense.