Hold your horses Mel_Traveller that is a sweeping statement & just pure guess work on your part and I'm 99% certain that is not the case. When EmilyP said her parents were told "your seats have been sold to someone else" I believe the phraseology used was erroneous & highly likely that it was to accommodate J pax bumped from the same flight the day prior.
For example the people in question were told that they could go the next day in business class but as that flight was full up the front also where do you think those 2 J class seats were going to come from - thin air??? Enter the domino effect.
Load factors ex the US have been extremely heavy early October due not only to school holidays but also the cancellation of a QF8 DFW/SYD flight a couple of weeks ago when the outbound QF7 flight had an air return to SYD about 4 hours out so there's
14F/64J/35W/371Y
seats you have to find for an A380 load of pax stranded in DFW with no aircraft which would have put an enormous strain on capacity on all other flights ex LAX.
It's easy to just assume that oversales are purely as a result of selling seats you don't have when the majority of cases as I've suggested above are trying to manage disrupted passengers due to a flight cancellation. It can also be when pax are inbound from another carrier eg AA that's either late or cancelled and they misconnect with their QF flight & need to be reaccommodated.