Perhaps something changes about the keys, like a base pattern that corresponds to a different set of physical constants. This could have a large effect on the properties of each universe or could be minor, depending on the change. At any given time, we perceive only a single universe, a single key on a chain. However, the other keys are here too, so long as we don't attempt to measure them. When we measure a particle, we fix one groove on our key, thus separating our universe from universes with a different groove.
Perception is the key.