discrepancy of backlinks from google

why do some sites have backlinks registered with google and others don't

[vkimura]

My friend has a site with google backlinks which are a page rank of less than three. Some were two and I believe I saw a few that were even a page rank of one. Does this make sense? Is there something wrong with google's algorithm?

Furthermore, I have around 15 pages with page rank of 4 with .php extension and all are linking back to my home page but none have been registered by google as any backlinks. Why is this? My friend's site's backlinks have many pages which are dynamic with .asp?id=xx extentions so why won't google even recognize my simpler .php extensions? If I created them as html will they not be registered? I created some to test what google's algorithm would do. I'll let you know what happens if I get any backlinks from those html pages.