Look in wikipedia for PR formula, you will see that depends on the number of outgoing links on the site referring to you too. In other words: you can be referred by a PR8 site and get almost nothing if that site has a lot of links.
The way google updated the PR's today makes me feel that they don't have any specific algorithm for this.. I saw so many good sites with lots of backlinks ended up with lower PR's and some waste sites got higher pageranks