I was flying US recently at an airport where TSA security lines were particular bad for that time of day. Flight was zero'd out, as it was oversold by 6 (info given to me by ticket agent and gate agent); however, 10+ people didn't make it through security in time, so a handful of people made it off the standby list. Certainly not the 20+ you mentioned, but it certainly does happen that standbys can clear a "full" flight.