It's a question I wouldn't have asked 10 or 15 years ago but now I notice it's being generalized... at least on the search engines I'm using or have tested, namely Google, DuckDuckGo, Yahoo and Bing.
I grew up with the known fact that a search engine by default performs a lookup based on all of the terms you put in a query. More than a decade ago, I could easily verify it, i.e. there was more than 95% chances results included all of the terms I queried.
Nowadays it's become rather trivial to demonstrate results do not include all of the terms but only some of them. FTR the first time I've raised the issue by my team I was first laughed at... until I demonstrated was I was talking about.
The usual counter-argument I was exposed with is to add the boolean operator "AND". It's easy to demonstrate that it doesn't change anything. Neither does an advanced search using the "Find all terms" input box (Google)... just because search engines, by definition, are supposed to use all of the query terms by default. But most often than not only part of the terms appear in the results. It's rather easy to spot results that do miss at least one term.
Of course in these cases the terms do exist in non-visible text elements (e.g. hyperlinks). However I did happen to find results that lack at least one term even from the HTML source (no example to give at the moment, sorry).
It becomes even more obvious with more search terms. The more terms the more probable it becomes to get "irrelevant" results. The consequence, at least on my own, is I stopped a long ago trusting the first result and often wade past the first, second and even third page of results.
Even though one could argue terms may still be found in non-visible parts of a page, the consequence is an ever decreasing trust in the results. I have noticed I happen to not find what I'm looking for even after the third results page. And not because I mistyped (which is also another annoyance of crawling engines, thinking I mistyped and spontaneously correcting my spelling, not even bothering to ask "Did you mean xxx?").
Why do search engines lie?