.... The vessel was rushed into service as evidenced in ST V, with even some very basic systems incomplete and not yet spaceready. In order to save a great deal of time and bypass standing contract timelines, turboshaft segments fabricated for new spacedock construction, and already delivered, were re-purposed and installed aboard the Enterprise. The numbers seen on the walls reflected segments installed in random order, since the labeling had nothing to do with the starship's deck structure, and were originally intended to represent spacedock levels. There were, in actuality, only the fourteen or so decks in that one shaft, though the now-meaningless labels read otherwise.
Mike Okuda once told me by phone that this explanation worked for him, so that's my story and I'm sticking with it.
Lora