>but the home page of that particular site has a PR of 6 and 2,290 backlinks.
And that, IMO, is an important key to the whole mystery
Check the homepage PR of all the bunkum pages that appear in the new, improved Google algo. Notice anything in common?
If looking at subdomains, check the core domain homepage PR!
My interpretation?
Use semantics to find the "core" phrase/topic. Identify what are the "authority" sites for that core topic by looking at the PR of the home pages of the sites returned. Take the top x thousand most relevant above a certain set PR level. Apply standard algo to them. Unfortunately, the top x thousand in perceived authority may have little or no mention of the targeted keyphrase but may only mention it in passing. Voila
Occassionally you may have sites that have got huge amounts of links and used standard SEO methods. They will appear untouched, whilst all their competitors fall out of sight!
All the various changes we've seen since November have been tweaking with the PR level needed to determine if sites are included in the sub-set of results.
Quite a neat idea really - if they could get it to work! Watching my sites I've been able to get a handle on where the cut-in/out point works with every tweak. So it works for me anyway - and it is nothing to do with on-page SEO!
As in all things, just a theory - but seems to do the job at my end!