While non-revs can show up on both lists, it doesn't appear to be the only answer.
I posted the same question up-thread (i.e., why is someone on both lists). I thought it was odd since most of the people who were waitlisted for F had cleared the C list >24 hours before the flight. If they were non-revs, they weren't space-available non-revs. This was on a domestic 777 (IAD-LAX).
One of the people who cleared into C and was waitlisted for F was sitting next to me. When he boarded he had a yellow handle wrap and Global Services printed on his boarding pass (he set his BP on the column between the seats where I could easily view it). So while non-revs can appear on both lists, my anecdotal experience says at least one GS also does.