From what I have read on EAS and drex files, that picture was taken when the model was being redressed from the Zhukov to the Excalibur. and apparently it was an intentional modification that was never finished, they apparently meant to lower the other nacelle as well, but never got around to it. I haven't been able to varify it, but from what I have read, the model is still in this configuration.
What I figure is that there was some damage, either from stage handling or left over Yesterday's Enterprise, on that pylon that couldn't be painted over, There is evidence on other parts of the model that they moved parts, like the saucer, to cover some damage. I suspect they ran out of time rehabbing the model and had to rush it out to filming before they could finish, which also explains why it has markings for two different ships on it. Since the last time we saw it was in Redemption II. I suspect they never bothered to fix it because they didn't need it again.
