I’m finding some strange results as well. For a site that I have done significant linking and work to – no PR change at all (across any data centers) as of yet.
For a couple of other sites that I have done NO work to in a long ass time – went to PR5.
For yet another site that I have done A LOT of work to – PR2 to PR0 as the update shows right now 
I think it is too much ‘in progress’ to tell much of anything at this point.