losangel - The SEO community has a lot of different viewpoints on this, but some of the most popular include:
Some links are manually devalued by GG
This includes pages like
www.statcounter.com, who by all rights should give each of their advertisers an instant PR 7 or 8.
Links that haven't been around a certain amount of time don't pass PR
The idea being that if a link is 'for real' it will stick around 6-9 months. Some people think that certain websites are 'targeted' for this time penalty as well.
Newer sites don't get the full benefit of PR passing for some time
This concept would indicate that Google is trying to prevent or hamper the efforts of new site owners from getting massive PR boosts through purchased links.
PR isn't passed between sites with too large a PR discrepency
This theory conjectures that a PR9 site linking to a PR3 sites shouldn't give an instant PR8, so GG devalues the PR passed. There is speculation on how far reaching something like this might be.
I personally recommend thinking about all of these and using your own best judgement. I think that some of these theories are very unlikely, while others have the potential to be true. One thing is for sure - I take them all with a grain of salt until I see lots of evidence to back them up.