Are you sure they're Google Ads? If you followed a link from the search results, and that link had ads from another search network, that other network would've associated you with that search term.
or the same scenario, but with doubleclick ads. The FTC(?) agreement for acquiring doubleclick included a clause that they can't mix google account information and doubleclick logging, but as parent said, the sites you clicked through may have had doubleclick ads and associated that topic (or their brand specifically) with your doubleclick cookie. Deleting it should reset it.
