But does this happen because Google is ranking those results above ones that matched both words, or because nothing on the web matches both words? I'm often unsure what's going on when I experience this.
There used to be a time where if you typed three words and got zero results, it was "no page on the ENTIRE INTERNET contains those three words." Now it's "these are what the algorithm thinks you want".
I think there's historical reference as well. If a site X publishes your exact terms but hasn't posted in a while, site Y that posts regularly appears to me like it'll be above it, even if it contains only a vague interpretation of your search term.
Generally speaking, I can find the information I am looking for, usually by forcing Google to include those words.