An intriguing thought. My hunch is, though, that rather than one factor, there's a "Supplemental algo" that determines what goes into that index.
Google has said that the core factor for Supplementals is low PageRank. I know of many high PR pages that target some very rarely searched phrases (eg, cutting edge technology), and I don't see them going Supplemental.
"Supplemental" is probably too strong a word for the kind of seasonality and burstiness factors that the patent most probably is used for, but these could be part of the monitoring continuum that includes Supplementals at the low end.
Google likes solutions that scale, and, as "all of the world's information" continues to grow, they're going to need to use every scaling trick they can patent.