I used to obsess about this months and months ago and today I hadn’t even noticed the update. 10 months in now PR4
But.
Looking around, it looks like a strange update. One site owner, 1 month in has gone to 5 or at least that’s what he reports.
Strange Things Are All Around
PR update cycle is 3-4 months. Some webmaster claim that, “my new site get PR2 within 2 weeks” it is because the site is close to PR update cycle. Some of the site that purchase huge links and couple of domain redirection, that also can instantly improve PR.
Also if someone is lucky enough to get a few backlinks from high PR sites then this can also jump the pr from 0 to 4-5 in one go. So without knowing the exact history details it becomes hard to conclude what could be the reason.