I believe that Google's user experience metrics are mostly based on the "long click". If users are happy with the search result, they don't come back and click on something else and they refine their search. Based on this particular experience, I suspect that you need to build a long click history in a niche to be able to rank for fat middle and head terms.
Google could do a lot more to measure user experience better. I have a five year old site that is now competing very favorably with large established sites with huge backlink profiles that were launched in 1995. I know from usability studies and from emails from our users that the site is better than the competition. However, Google doesn't see that. In the last two years we have made huge gains into fat middle rankings, but the head terms all go to the huge old established sites still. Without improvements from the competition, it should be a matter of time before Google is able to measure our site as the better one. I just wish they could do it more directly and faster.