The Death Star was pretty far along for it to be before Obi-Wan dropped off Luke. That's why I always figured that was a "timeless" scene. I never thought the ending was in chronological order, it was just a rap up of where everyone ends up leading into the next film.
That was one thing I really didn't like about the ending of ROTS. Based on the timeline established in the original trilogy, construction on the Death Star shouldn't have started until a few years before ANH. Maybe the one seen at the end of ROTS was a prototype?