Why is there a PR diference between a Forum and its subboards?

I searched in Google and found some high PR forums. As I open the first page, SEO tools bar on firefox shows a PR value of 3 or 4. However, as I registered and clicked one board. there is no PR value at all. So queer.
Have anyone ever met the same  problem?
Is PR only based on pages rather than board or website?
if it is, is it still useful for me to post in pages with no PR value at all?

Thx in advance.

PR is unique per page. You don't need PR links, but relevant links from relevant content