It became an interesting and repeating observation in recent months. Site A has PR 7 backlink and 3 backlinks total. Site B has PR7 backlink and 600 backlinks total. Site A got PR6 while Site B has only PR3. This almost implies that the pagerank flowing into the site is 'averaged' over all incoming links. Meaning, the larger number of crap links (low PR links, no Pr or PR 0) you have, the smaller your resulting pagerank is as these links devalue any high PR links you might have.