How do sub-pages get PR so quickly?

I posted a new thread this afternoon here on DP, and it's already got a PR of 3. How does that happen so quick? Is it because this site has such a high PR for it's main pages? My sites, which are PR3 & 4, don't get PR on the sub-pages (forum threads) for weeks or months, and sometimes not until there's a new PR update.

 

It's weird as the main page was indexed two days back

 

This thread already has a PR of 3. How'd that happen so quick? What's the main page being indexed got to do with it?

 

...because all the posts are actually the same page, just with parameters added.

 

http://forums.digitalpoint.com/showthread.php shows PR4, this thread http://forums.digitalpoint.com/showthread.php?p=71024 shows PR3.

 

I assume the page without any parameters is one PR, and then all the other pages with parameters are another. Just guessing, though, on that one.

 

That is indeed really weird.
I think Google just looks at the number of hits that are made for a particular page. That leads to high PR.

 

Hmmm... http://forums.digitalpoint.com/showthread.php is PR4, http://forums.digitalpoint.com/showthread.php? is PR3. The only difference is the question mark at the end.

I was just wondering, that's all. I thought maybe somebody knew for sure.