The PR of backlink also plays an important role in Site's PR, Good PR cannot be achieved by just creating backlinks in huge quantity.
Hi ... I have a friend, who's web site is a PR3, with 9 internal links and zero externals. All 9 pages are bios of employees, which never change. Click here for the 9-page site. Ugh!!!
Sometimes it also depends on the site it is hosted. One of my site has PR3 which I have never done any SEO or got any backlinks for it. Here it is : createatlantis.com
It happens because of two reasons: 1. The site only have a few external (going out) links 2. The PR of the backlink is high.
Their 6/10 links are come from invision power community so I would assume this is the main reason for their PR. Others link only has PR1 each.
I have 200+ back links and I'm still only PR1, my website is only 4 months old, but in a years time i will still be lucky to be PR3