Yes, I've noticed this, mav, and puzzled about it for a while. In the end, I have dismissed it as on of life's ineluctible mysteries, like why is it that when you press the button on any given controlled pedestrian crossing, sometimes it changes within 30 seconds, othertimes you have to wait for eternity before the little green man appears (and this is when you are the only person who has used the crossing within the last ten minutes)