It randomly selects the keyword from your page and shows the ads. I don't think it is specific to your tags.
Alright!!! Finally a thread that is not about the PR update!!! Woohoo!! They go by everything. Page title, description, keywords and most importantly content. Page title can be important if your talking about Adsense. I have a few blogs that used post numbers for page names and they got public service ads. The I switched over to post name permalinks and they instantly switched to relevant ads.