I think it’s because some Microsoft computer had a dedicated button so you could do things like Office+W or Office+P (or apparently Office+L), but they had to make it use an actual keyboard input so they used a string of modifiers that wouldn’t be used by accident.
Maybe there’s a cultural idea about mirrors being somehow “the same”. After all, a mirror shows the same thing regardless of which one it is. Or related in cultural mythology to a singular adjoining world that contains your doppelganger (in such media, you don’t usually have a separate mirror-self for every mirror, but one that can be accessed from any mirror). Also could be a turn of phrase that stuck without a good reason.