We started to elaborate a bit on PageRank theory in this thread
The thing about the amount of link returned buy each search engine is they all do more or less what they want.
Google knows of many more links than the one he takes into account, Yahoo seems to show the complete amount of known links, and MSN is in between.
And this does not tell us anything about the way they actually use these datas.
If yahoo! tells you've got 2000 backlinks and Google only 100, it does not mean you'll get better ranked on Yahoo.
About Google PageRank prediction, it's a prediction, so it' not like it will never be wrong.
It's Live PageRank, the Pagerank you'd have if the DC updates would occur right now and it is always moving a bit.
The interesting matter about this tool is to follow the evolution.
If you've got a big difference between the PR you have and the one predicted, it's telling you in which direction it should end up to go
Then you'd better have 100 backlinks from 100 different domains than 100 on a single one.
Google generally counts more than one backlink when a web site is back-linking on all pages, but will not show all.
It's obvious there is some dumping.
As well, good backlinks are the one coming from highly ranked pages and the most related to your content. They can link the home or be deep backlinks, both are needed to build up a nice ranking