I believe it relates to how the internal and external links have been set up. IE if 100 of your external links point to
http://casinoaffiliateprograms.com
and 25 External links point to
http://www.casinoaffiliateprograms.com
then I would assume that the first url would have a higher PR. I believe this would also apply to internal links.
It’s just a theory and I could be wrong. Anyone else have any info on this?