It has been pretty well established that there are two PR ratings. One that Google uses internally that can change daily. The other is toolbar PR, which is only updated about once every three months.
Originally Posted by SEO Service
However, we do not have any way to get at G's internal PR to prove or disprove this. I would suspect that most of the time they are approximately the same, except for a new site that is gaining PR.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin