It's strange they didn't consider this Bespin continuity nightmare in the first place, the drama in the movie is on it's peak, so the most people didn't care where it all exactly happens ... but still...

I had the same way of seeing this thing as Imperial Fighter says. Carbon Chamber was for me somewhere else in the city understructure, near hanging "ramp" incl. round window chamber where Luke and Vader continue their duel.

In the movie it's not specified how many tunnels Luke uses to get from the carbon chamber to that round window chamber. Theoretically there may be more different tunnels Luke uses - that would be an "exuse" for the wrong position of a round window chamber regarding extended arm access tunnel which Luke entered and exited in the movie.

One simple way to lose this problem could be a different lighting for tunnel during entering vs. exiting scene. I can not anticipate if this would be convincing solution.

Another could be, as Adywan said, to make him get out from some kind of elevator instead a tunnel. Adywan?