Google doesn’t “keep its PR” high. The PR formula is part of a computer program they run–their algorithm. No one at Google manually updates the PR unless there’s a penalty going on.
Also, the toolbar PR is only a snapshot of the site’s PR from the last time they updated the toolbar. The actual PR that’s factored into sites’ rankings is being updated behind the scenes on a regular basis.
It’s just a factor of how many sites are linking to the domain and how important those sites are.
Also, if you want to check backlinks, Yahoo will give you a better picture than Google. Google has deliberately broken their backlinks function in order to reduce the possibility of SEO manipulation.