I have to be honest, I have wondered the same. More out of curiosity than anything. I would just like to see what takes and what doesn't, since my site is new.
I believe the backlink update being behind the pr update is actually one of the signs of the apocalypse. The end is nigh, repent your souls.
I think google check each backlinks first if it's quality then after that they would decide what PR they would give on the site.
Good for you... but then most of us are still waiting for backlinks update...Google is so slow on this..