The large ad is a traditional Google Adsense ad. The site owner has little control over what gets shown there. Google's spider scans the page for keywords and tries to match ads that match the kind of keywords that are on the page.
A forum site often shows things that are very off-topic because the site owner has little control over the "elements" of the site. In a traditional site where you are targeting specific keywords when you write an article, those keywords will be purposely infused into the html Title Tag, Description Tags, Meta Keywords, H1 Tag, somewhere in the first and last paragraph of the actual copy, in the actual file name of one of the pictures on the page as well as the Title and Alt attributes for that picture. This is all just good SEO. When all this combines together, the Google Spider can see what the page is really "about" and adjust accordingly.
Unfortunatley, with a forum, the site owner has no control over SEOing out each thread and so whatever people post ends up being what Google has to work with. This means that very often there can be no continuity in the ads.
For example if the spider sees that the entire site is pretty much related to Guys > Guns > Gun Reviews > Springfield Armory Gun Reviews, Springfield Armory XD Gun Reviews ...then it can tailor displaying ads for XD Guns and XD accessories.
If, however, the site has topics all over the board then it only gets to Guys > ....and will display ads related to "Guy Stuff". You have to actively turn on certain types of ads so most guy sites will screen T&A ads...but other than that you can get everything from knives to Super-Charged Snowblowers if the Google Bot just tags your site as a "guy site".
