Egol, moderator at SEOChat, gave me some homework to do - post on Sites That Defy Gravity. These are sites that for no apparent reason, do not drop to the bottom of the rankings, even though from all appearances, they should not be ranking well for their terms - but they are. My personal belief is that these sites have at least one ultra-powerful backlink (perhaps more), that is as yet undiscovered when one is evaluating the reasons. I know when I evaluate a site's ranking, I rarely spend the time it would take to look at every known backlink. Instead, I glance through the list of backlinks, randomly pick some that I think might be having an effect, and look at the pages where the backlink appears. Somewhere, in the known or unknown backlinks, is probably a source that Google considers to be a mega-vote for the site. That mega-vote keeps the site in the SERPs even if we are unaware of it.
Of course, there is always the possibility that it is nothing more than a fluke. 😉