I suppose the difference here is that no character ever declares the rear wall of the observation lounge to have always been that way. Compared to...let's say, the wall of main Engineering (the bit which is a corridor plug) that was open in early S1, then closed for the rest of the Tv series, then open again in ST-Generations. In one episode (I forget which) one of the ship's designers came on board and stated that he remembered running a conduit behind that very wall. Now THERE's a mystery!!!