> I have no idea what kind of metrics lead Google towards fixating on the most generic terms of a search

Total shot in the dark, but I'm guessing that interpreting a query string more generically allows Google to serve up a broader collection of ads.

There used to be an operator in search engines of yore (including Google, iirc) that specified, basically, "expand beyond exact matches" to include synonyms and such, maybe "~". Google has basically inverted that, so exact matching requires explicit input. I also find it frustrating.