Google sees it as two different pages so they have different page ranks.
You should be able to see a difference as well on:
domain.com/
and
domain.com/index.html (or whatever page is directory default thingie).
The PR you see on your toolbar is purely for entertainment value. Google's "real" PR for your page isn't published.
Worry far, far more about on-page content, inbound and outbound links than PR.