Just a note, one of my sites moved from PR3 to PR4. It was a PR4 before the last toolbar update (a few weeks ago, I believe) but moved to PR3 during that update. Just thought you may want to know.
His toolbar might have just been looking at a particular datacenter. One of my sites jumps from 2-3 when I use the toolbar during an update.
Google updates toolbar PR every 3 to 4 months. I believe the most recent one just finished a week ago so that means another 3-4 months until the next update. However, PR is updated more than 3 to 4 times a year. So when you see a change in PR in case the toolbar updates, that doesn't necessarily mean that it is your current PR, It may have been your PR since it was last recorded. Your PR value is regularly updated but it just doesn't show in the toolbar
The last update hasn't settled on all datacentres yet, it's possible you were puling PR values from a datacentre that hasn't updated yet.