I don't think it works like that. Otherwise how could there be so many more PR10 sites before? You're right that its exponential of course.
There are lot of sites with pr10 but hard to find them not unless if you want to spend time to find them.
Actually I think there's barely any sites with PR10.. Google, CNN, Adobe Flash Player and that's about it. I would never expect any site of mine to hit PR10, I would be happy with a PR7 or PR8 website!
But Google is not on the top of PR10. According to this site: http://www.seocompany.ca/pagerank/pr-10-pages.php The #1 rank on PR10 is Adobe.
The only sites I know of with PR 10 are cnn.com, usa.gov, w3.org, and google.com. There's probably a few more others but those are the only ones I know of.
Its is one of the top news sites around the world. http://www.cnn.com/ top site for reporting frauds waste and other things - http://www.recovery.gov W3 consortium http://www.w3.org/ Science direct - http://www.sciencedirect.com/
Most news websites have very high PR, even though none of the news articles are written with seo in mind. It's due to the nearly every-minute updating, the million of readers and the backlinks that they can get very high PR.