SCANNERS?

I rarely travel by air but I recall being puzzled when I walked through a metal detector and set it off.  Because I had walked through others that same day without an alarm, I initially couldn't think of what was causing the alarm.   After being threatened with a strip search, I tried removing my calendar book:

I guess the little metal rings were enough to set off this scanner but not the other ones, which raises the question of who calibrates the scanners?