I must have just not noticed the machines at the other gates. It was only when I got to my own gate that it struck me.

Given the number of aircraft/passengers in the airport (!) and the number of gates provided, looks like a shrewd idea by an x-ray machine salesman. But the more I think about it the more it makes sense. It also gets round the trend in some European airports to put you through the machines twice, once at the beginning and again at the gate.