Getting back to the original Trek Door script, everything becomes a lot clearrer when you un-flatten a door.

Getting back to the original Trek Door script, everything becomes a lot clearrer when you un-flatten a door. In the image below, you can see that it's actually a PRISM, which is created by tapering a hollowed box. When the door opens, it reduces the tapering to 0, and when it closes it resets so that those sides meet.

This, then is why you get two faces on the front, and one reflected faces on the back - the back is only one face, repeating.