if it are not user metrics what else could it be?
A number of things, including the relative level of co-occurring phrases used in the wording of the page.
What I can say is that if you have terrible user metrics (from search engine traffic) you don't get much visitors. The other way around is not always the case.
I would definitely keep in mind correlation is not necessarily causation -- "Everyone" says "if you increase user engagement" [manage the numbers]
rankings will increase, but the numbers aren't, IME [In My Experience]
, the cause/effect. What's more important, again, IME, is giving people what they are looking for in a way they can find easily.
One thing to keep in mind when reviewing numbers is: Google has stated it's interpretation of it's data indicates people return to the results and keep searching even after they have found the answer to their question. Granted, Google has more data than I do, but I have yet to find a single person who actually does the preceding, even though that's what Google interprets the data it has as indicating -- The people I have spoken with stop searching
when they find the answer they are looking for.
The "take away" from Google's interpretation of the data is: Search, click, return/search again becomes much more "noisy" as a signal and much more difficult to use as a direct ranking signal due to the way they interpret the data they have access to.