Can we really say uncentered, based on the barely-in-perspective art that actually appeared onscreen? If the artist merely left out the "-15" when going for coffee after doing the "NCC", and subsequently went home angry and tired because the coffee machine had broken down, we'd get pretty much what we see. Plenty of room for that "-15" before the next row of portholes...
Actually, I could well see one of the responsible artists putting in "NCC" and then deliberately leaving a space for some numbers those interested in such things would wish to pick for the ship. And then such a person indeed coming in and squibbling some random numbers on the nacelle while not noticing the saucer thing at all.
Timo Saloniemi