If your site has more than one page - for example if it's not:
yoursite.something, but
yoursite.something/page1 - yoursite.something/page2 - etc.
then the PR of yoursite.something and the PR of the yoursite.something/page1 (and/or yoursite.something/page2) may be different.
I understand it in this way. Let other SEOers write if I am right about it. Thanks.