Another data point...
Last week LAX-JFK 9:30 pm red-eye flight was oversold....
Went to the gate and did everything right including hanging out at the gate.
Towards the very end of boarding the GA signals to me and the two other people who came after me and asked if the GA need volunteers.
She tells us all to board as there is a party of FIVE that has not even checked in yet and as it is now 15 minutes before departure she releasing their seats and those five seats means that she won't need us as volunteers
She did thank us for volunteering....in hindsight I could have at least asked for a HOOU (Have One On Us) certificate...
The point is that sometime even if they are oversold people do not show up which is why they oversell in the first place....