On a point of order, Groundhand, I think you'll find that the airline, directly or via a handling agent, is responsible for making sure that scales are correctly calibrated. The passenger's contract is with the carrier, not the airport nor indeed the handing agent, and the weighing and charging are carried out under that contract.
The airline must take the necessary steps to ensure accuracy, including checking the scales and calling the airport to fix them if wrong if the airport has leased them. And the airline must ensure that its appointed agent does exactly the same.
There is nowhere for the airline to hide on this issue, which is a serious one of fraud, and is very common, deliberately and otherwise.
All the airline/agent needs is a handy object known to be precisely 20Kg with which staff can check the scales each time they take them over. It takes 10 seconds and there is no excuse for not doing it.
Trading Standards must be called whenever there is a reasonable suspicion that scales are faulty. Do not warn the airline, agent or airport, for obvious reasons. If you are right, TS will tell you and you can recover overpayments.
If we all do this without hesitation, this scourge of the industry can be eliminated. Over-reading scales are the norm in many airports, and the airlines that use them are well aware of it.
As light relief; I happened to glance just now at the T&Cs that Easyjet push out. Quote "the aircraft is boarded in priority order, so the earlier you check in, the more choice you will have!" The exclamation mark is Easyjet's, not mine, but I'm surprised, too. I wonder if the writer has ever been on an Easyjet flight?