How often do you think the PR updations will happen?? A PR is generally given based upon the number of back links and also considering the quality content on that page. So do you think PR should be high or just a balanced one??
What we call a "PR Update" is a snapshot of the PR that was taken on a particulair day and exported to the toolbar.
Google uses internal PR as a factor to rank sites which is different than the Tool Bar Page Rank. Think of is this way, internal page rank is a suace that has more ingredients than the simple TBPR.