I'm not saying it was "artistic greatness," I'm just saying it was an attempt to add a little variety rather than the result of incompetence. The premise that it was incompetence is predicated on the assumption that the only legitimate way to assemble a model kit is exactly per the instructions. Those very kitbashes you mention prove that the art department of the Trek shows did not hold to that assumption.
Besides, everything that goes onscreen has to be approved by the producers and director. Okay, sometimes mistakes get through, but there is a process to catch them. In order for this variant to be a mistake, it would have to be multiple people
's mistake, and I think that's an implausible premise. It's immensely more likely that it was an attempt to be creative within the limits of time and budget. I'm not saying it was brilliant, I'm just saying it's unreasonable to assume it was a mistake.