Their report states this basic fact:
"Here's how the PageRank is determined. Suppose that page Pj has lj links. If one of those links is to page Pi, then Pj will pass on 1/lj of its importance to Pi. The importance ranking of Pi is then the sum of all the contributions made by pages linking to it. That is, if we denote the set of pages linking to Pi by Bi, then [picture]"

The whole analysis-type article is located at
ams.org/featurecolumn/archive/ ...

Thanks for the news Juha-Matti