If you study pagerank closely, you'll notice that pages which update often have their
PR increased much easier... For instance, new sites with fresh content daily easily see increased pagerank vs internal directory pages which used to see the trickly down effect. One of the things Google wants to try and fight back against. You'll notice internal directory pages loose their
PR or have reduced
PR if the pages do not get updated or individually linked to.... There are many factors involved here.
PR doesn't trickle down like it used to...
Long ago, having a PR7 site link to you was almost a gaurantee that your page would go PR6, but this is no longer the case, mostly because of the whole buying links game.