One can also make keys and locks with some tumblers/notches nonfunctional. That is to say, for a particular lock, it might not care about the depth of the notch on a particular position on the key, but another lock might care about that notch. We have keys like this in my department: Grad students are issued keys which will open the doors to the building and to the grad offices, but nothing else. Those grad students who work in labs have a key which will open the building, the labs, and the grad offices. Faculty have a key which opens the building, the labs, the grad offices, and the main office, plus their own individual office. Folks who work in other departments in the building have keys that also open the building and presumably their own doors, but none of the physics department offices or labs. I suspect that the locks on the building doors only actually regard one or two of the notch-positions, ignoring the rest.
All of these keys look very similar to a casual glance, though if you hold them side-by-side and compare carefully, you can see slight differences.