Thanks for that, lammert - it certainly does support my idea.
I started thinking in this direction because of baberjaved's question in another thread: Removing Low Ranked, Un-useful Pages - worth it? [webmasterworld.com] and Matt Cutts' frequent advice to let googlebot crawling and indexing be as open as you possibly can.
Add that to the difficulty in getting a stable number for indexed pages [webmasterworld.com], and it seems to me that what goes on with Google is a lot more than what we see, or even what we've guessed.
URLs that are in Google's back end bur not publicly visible could be Google's own version of dark matter [en.wikipedia.org].