A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
If it's same elite level, fare class, etc., the difference could be the time of upgrade request tiebreaker. One or both of the passengers could have requested the C+ and FC upgrades at different times. Of course, this is unlikely and even more unlikely if there are other people between the two on upgrade lists.
My bet is RUC usage for only FC.