It's common that I get search results that don't contain the keywords I searched for. The results will also contain the keywords but in different parts of the page, so short phrases like hacker news will match pages with "hacker" in the first paragraph and "news" somewhere else on the page. Google used to recognize a hyphen as meaning "only return results where these two words are joined by a space or puncuation", now it ignores the hyphen and you have to enclose the phrase in quotes to get that behavior.
Even more interesting than a simple full-text search would be a regular-expression full-text search of the internet.
In either case, I'm happy to hear IndexTank is trying to fix this.