It is an ADR requirement that speedos be within 10% accuracy...that is correct. But this is only from the factory. After it has left the factory and been sold off the dealer showroom floor, the responsibilty is then with the owner of the vehicle.
Just like maintaining a car with roadworthy tyres, lights that work correctly etc, its up to the owner to make sure that his or her speedo is accurate. In previous years it was harder and more costly to check speedometers. It could have been done with a vehicle dyno, or at a police checkpoint every so often when they would put up a speed checking sign with a radar.
But now, there is no excuse with the price of GPS units that give you an accurate speed reading, plus alot of phones even come with a GPS which is another method of checking.
As with anything on a car, it is all up to the owner to maintain and check and make sure everything is working as it should and in roadworthy condition. Nothing is failsafe and we can't expect things to work perfectly throughout their lifespan.