Welcome to WebmasterWorld Guest from 220.127.116.11
I'm guessing that the "pagerank not assigned" yet pages are eitehr pages that have gone supplemental because of disappearing, or that the pages remaining just have a higher percentage of new content - did you keep the newer material and lose the old?
These are just guesses - somebody more learned here will be able to tell you more...
One thing is certain...those who have lost ground have also experienced a large increase in "PageRank not yet assigned" in their Google Sitemaps.
Interestingly, when checking site through the errant spider, they're right.....most of my pages don't show a rank yet.
But when going to other data centers, my pages are all ranked.
Yet in the sitemaps, it continues to report a large portion of my pages unrated.
And the plot thickens!