I believe the detectors at all airports are probably calibrated to activate above a certain threshold. But that will be in the factory. In situ, other machines, chairs, handbags, people, "Tensabarrier " stands etc. all contribute to the activation threshold. I pass through these wretched machines all over Europe wearing exactly the same clothes (funnily enough a uniform) and believe it or not, some machines activate and others do not. It's not worth a discussing the activation, you just play along with the pointless charade. After all, as long as I don't have nasty pointy stick or an aggressive yoghurt they'll give me a plane full of people and which I can fly where ever I wish at lickety-spit speeds. You couldn't make it up.
PM