I think that all the search engines assign a score to each website, for fixed factors that don't depend on keyword searchs, i.e. depends on links, domain age, traffic, quantity of content, reputation of domain registrant if any
Plus another score, dependent on the query run,
Perhaps the two scores are then merged
A site which scores very high on the fixed factors, might well do better than it should when the overall scores are computed,
ergo, bbc pages about programs about my niche used to out gun me in searchs in my niche, twas quite sad actually
Just my guess work