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Note that is an inference on my part rather than an explict from them!
Standard 3 column vanilla pages, table layout. Center is the content. Right column is AdSense. At the bottom of the content, there's a line of text that contains a keyword that's off-topic. In source html, this is 4 lines above AdSense. The AdSense tended to target that keyword.
AdSense knows the general theme of this site (judging by the ads shown two weeks ago) but is now confused about the page. Deleting the keyword has given some of those section's pages much better targetted ads.