I've seen some wierd stuff too .. I had one page on my site have a sudden spike for no reason at all .. it's a little links page with only one link to it from one page in my own site. For a few days it out-ranked my homepage.. blew my mind.
Although some DC's have a different guage of your site, I doubt that there's a reason that they would PR5 & PR7 difference (given that PR7 is quite difficult to achieve these days). I would try a link: search the next time you see it, as IMHO incoming link updates are the only likely force that would have made such a difference.