I have a pile of new sites that all gained PR this update and all of these sites back linked to a PR 2 site (this was the only outgoing link across all the new sites) Each new site achieved at least a PR3 yet the PR2 site they all linked to remained unchanged this update. Surely with so many incoming PR3 and PR4 links it should of gone up to a 3? It even has a PR 5 backlink and yet it remains at PR2
It's very strange but the same thing happened to my site (where one wasn't even worked on.. backlink wise and it raised a PR while the other one did not). I guess you need a larger amount of backlinks to bring it up.
Yes, that could be it. But it could also be that google is discounting the links to this site, because they look 'unnatural'.