it's possible that your master code is 3-1-3-5-7, or 5-7-9-9-7... running your finger back and forth over the key pad is like running your fingers over a keyboard... eventually, you might find a user whose password is 'qwerty'... it's not impossible.
If, in fact, it is 1-3-5-7-9 or something similar, it's entirely possible (as I stated before) that someone programmed in an alternate code at one time and it was never erased. In fact, I've programmed that exact code in mine when I go camping just to not need to carry the keys around for the weekend. Of course, I promptly erase it so I don't have to worry about it getting cracked by a 5-year old who likes to push the buttons!
Take the time to erase all the user codes first, then try it and see it 'zipping' the keys still works.
-Joe-