As you will see when you click on the "key" graphic, some "keys" can be very elusive. For search engines, most people say the key to winning the popularity-usage competition is in the amount of RELEVANCE that is perceived by the users on the first or even second SERP.
The first challenge, though, is that for many years philosophers have known that there is a "Problem of Perception". A search engine user may look at a "keyword" along with some short text in an organic or paid listing, and get the illusion that it is exactly what the user is looking for, only to find out that the author of that text had something else in mind. The second challenge is that what is perceived as pertinent, connected, or applicable regarding a given matter to one user is not to another user. Relevance is very personalized. The third challenge is that even personalized, relevant SERP results must be time sensitive, as people change and evolve over time. So what is relevant in the past to one search engine user may not be relevant in the present or the future.