Do your ads from Google Adsense sometimes seem to have little relevancy with regards to the content of the page?  Generally, the keyword density in an article is enough to trigger the correct contextual ads but sometimes this just doesn’t seem to work.  I have had a problematic page recently that would show generally the right ads but not the specific context I wanted.  I changed the <h1> title, changed the first paragraph and increased the keyword density with little result.

In desperation I tried changing the meta keywords as these were a little general.  I didn’t think that Google paid much attention to meta keywords anymore but unless this was pure coincidence it seemed to work.  Maybe the Google AdsenseBot does look at these tags?