I just launched off a new forum at www.AffiliateHelpForum.com and my Adsense ads are constantly generating "blog" advertisements. I don't use the word "blog" anywhere on the site. I'm sure many of these advertisers are looking for "targeted" traffic which in some cases is not what they're getting.
After you put adsense code, you need to wait for a while in order to see relevant ads. Anyway, if your problem isnt solved, then use these commands to tell google what content to use for displaying ads. <!-- google_ad_section_start --> <!-- google_ad_section_end -->