Korrd - 6:32 am on Feb 26, 2010 (gmt 0)

This is filed in my brain as very old knowledge so I don't remember why I think this, nor do I know if it is still applicable, but my belief has been that when Google counts way more matching pages than are displayed and less the standard 900 plus are shown it is because pages containing multiple occurrences of a query match were pulled more than once from the index during the retrieval process. The duplicates get sorted out during the ranking process leaving less pages to be displayed in the SERPs.