I've dialed back SFA RB performance a few weeks ago. A SFA HB could still rip off a 60 yard run (with lower probability than before the adjustment), in which case their YPC will still be good even if every other run in that game is less then 3 yards.

All NFL players have good games and bad games. So it is possible that a SFA will have a good game from time to time. In a small sample size it is easy to sample well above the mean performance of a given player. Context also matters. If you have an all star surrounding cast, a FA can look decent. But averaged over a bunch of games, SFA are going to perform worse within context that average players.