The ads that show on the site must be on topic for anyone to click on them, the visitor to your site is there for that certain subject your site covers. I have found that by changing the type of content on a page and altering the keyword density significantly alters the ads showing on that page. It seems that underlining and bolding keywords (but only once or twice) and correct placement of words on the page also makes a difference. If your site does not show the ads you want you must look at the set out of the content and remember how the engine will read it. This alone can make the difference between on topic ads showing and either non topic or even no ads at all.
diglet is just saying that you can manipulate what ads are displayed on your page via it's content and keyword density. This should be quite obvious as Adsense is a contextual advertising program afterall. Another great way to ensure you are displaying relevant ads is via section targeting. This is simply placing a particular section of your page in special comment tags, showing Google what the ads on this page should be about.
Just remember it can take a few days for adsense to properly target the ads that show on your site. I've seen so many people ask why their ads are not targeted a few minutes after making changes. For quickest results, it seems to initially google look at the keywords in the url before anything else.