For instance, I recently watched a terrible movie, and I'm trying to remember what it's called so I can warn people off of it. Searching for that is a specific piece of information, not a generic "noun"-type thing:
A query like "sports" or "news" may or may not have an ad in the search results. Keywords like "credit cards for people with poor credit scores" will almost always show lots of ads, even though the query is quite specific and contains 8 words.
In some particularly valuable verticals (like hotels) Google then further adds their other paid verticals to the results in addition to the AdWords ads.
One of Matt's videos (not sure which one) mentioned online publishers tend to focus a lot on say 10% to 20% of highly commercial terms, while often paying much less attention to the rest of the searches as the commercial terms typically are far more valuable than the informational ones. (Mesothelioma lawyers can afford to pay more for traffic than say people selling flour, or people offering recipes that contain flour).