This idea just fell on me - and although I can't think of a way to test it, maybe someone else can.
We've all seen URLs "fall out" of the index at times. But does Google still use those URLs as part of their web graph when they iterate link juice? Or is their web graph confined only to those URLs that can be retrieved as search results?
I'm sure that once a URL is crawled, the data is not just gone if the URL is no longer in the visible index - Google wouldn't throw away data, right ;) So I wonder if the publicly visible pages that can be shown in the SERPs at any one moment are only a subset of the full web graph that Google uses for PageRank calculations - and others, too.
What do you think?