I beleive they do alter the sensitivity depending on the day, if important politicians are traveling i've heard they bump them up a bit.
In response to the fact that you can go off in one machine and not another on the same day with the same gear, this all depends on whether you walk through the middle of the machine or not, If you brush up close to the sides with a watch etc it will go off, I've experimented many times, if I keep my watch centered I can go through with quite a lot of metal!
Cheers All