This is very simple. The interior of the elevator is a single room, but which is mapped east of both of its termini. The reverse map connection, west from the elevator, can only go to a single room, and that's what determines which floor the elevator is on.
UNCLE Headquarters is a room. "The steel nerve-center of the free world's battle against the Technological Hierarchy for the Removal of Undesirables and the Subjugation of Humanity. Being against technology, we have only a very simple elevator to the east."