It is interesting about the maps, but I read the headline with the context of Google search. Any SEO experts know if Google search also penalizes 'dead-end' websites (ones with no outbound links)?
Edit: Erm anything wrong with my comment ? This got voted down to -1.
As a graduate student I look into algorithms like these for ranking web pages.
This was with reference to dead-end websites and awarding a Pagerank like scores based on what my site points to (in addition to those who point to me)
I got down voted, I think because my comment was about web pages and it had got separated visually from the other comments about ranking web pages. So I added it in that my comment was about the discussion that had ensued about web pages.
Not complaining about the down votes, (and surely not about the up votes :) just that it is quite weird, and unlike anything I have experienced here on HN.
Since Pagerank is an expensive computation, one effective optimization/approximation is to cluster pages into groups and then compute the Pagerank per group. The group-scores are then re-distributed.
I am not sure whether this is what is going on, but it is a possibility.
Edit: just found one that is a 3, but it has a trailing slash while the others don't (so it might be considered a directory and computed seperately.)
It also just occurred to me that my xml sitemap assigns a priority to each page, and the ones with a PR of 4 are the ones with a 0.8 priority or higher. There's another top-level page with a 0.5, but it has a trailing slash, and it's a wordpress blog so it has it's own sitemap. So I think that 4 might be independently calculated.
Not trying to toot my own horn, but you might find the bookmarklett on this page useful, it certainly made things much easier for me: http://nfriedly.com/pagerank