It goes without saying that in order for Google Adsense to function with maximum efficiency serving up targeted ad content, the content on your website should be focused, clear, and be easy for your audience to read.

For example, this section of lumine.net/adsense is aimed at educating the site visitor about AdSense, how to use it to generate revenue, and how to maximize its potential and not information related to other topics.

Having said that, it appears that Google does have some pre-bias on what has already been spidered by the Googlebot and sometimes non-targeted ads will show up on the site. If I had to take a guess, it is because of the links in the header which take the visitor to other parts of my site.

Factors that should be kept in mind when creating content for your site:

KEYWORDS, KEYWORDS, AND KEYWORDS
As mentioned above, the most important consideration you have to keep in mind is that AdSense only works as good as the content on your site. This means that the content should be relevant to the topic, should include keywords that are topic-specific, and more importantly, should include high-paying keywords.