TBH my theory is they saw the official portrait about once, entirely missed the shading indicating the roundness of the cupola, and for whatever bizarre reason drew from memory instead of looking up references.
Would go a long way to explain all the weird fuck-ups in the design like, say, getting the suspension badly wrong - 4 instead of 6 wheels, complete omission of the underbody transmission elements etc. Or that Ass Pulled handle on the upper rear hull where the original instead has what looks like some kind of projecting machinery housing. Just to name a few.