Each and every detector should be the same, but they are not. They should be calibrated in situ with a standard target set at various heights. On the day they are tested I'm sure they come up to standard. But once the calibrator leaves, the variables set in. The goon holding a tray of metal objects, a passenger and their baggage near the sensor, barriers, tables and chairs are moved and so on. I'm also sure other environmental factors play a part in tripping the alarm as does 'wander' in the calibration. Best advice is go along with the show. Security personnel are not selected for their personal charm or ability to make a rational argument based on fact, just to follow orders. If the machine goes 'ping', just like one of Pavlov's dogs, they follow a prescribed process.
So yes there is a standard, but only when the machine was tested.