PageRank only updates every few weeks to months. It's not an instant thing, nor is it computed every time the index is updated. What you're seeing is due to a PR update this week not propogating to all 100,000+ servers instantly. Since the toolbar has a list of servers to choose from when it goes to check a site's PR, some were updated while others were not. You can use an online tool like pagerank.net to get the new PR reliably.
Anyone who thinks someone can alter your PR by takign away a link instantly hasn't thought about PR... it's a recursive function that has to be run over the entire 5 billion+ page index in order to be computed, because every single link has to be accounted for in order for the equation to balance (the result is 1, actually). In the process the PR of each page is computed. There are estimation algorithms that are faster, but computing PR likely takes several days on a cluster of computers.