I can't make myself familiar with these 40mm more at the bottom.
Why:
Imagine you're a prop builder in the shop. The box itself was - what I assume - built, then the lifecasted body parts went on. The carbonite texture was then made by hand (after that some bondo here and there, sanding and finally painting the thing).
If the plan was to not make it fully square from the beginning - for whatever reasons, maybe aesthetics (?) - then as a builder I want to make this unsquare / trapezoid shape clearly visible since it would be part of the overall design which I want the audience to see / notice.
A difference in the width of ... 40mm...intentionally...why? It would'nt be noticeable to the audience (set lights, visible screen time, angles etc.).
What these 40mm will do when constructing / building the box is, that it is not square. Not square means you have to cut things in an angle where an angle is not really necessary (designwise). In the following that means more time to build it (tbf not that much more time).
Another thought:
Maybe these 40 mm are a result of not building the box as a solitary piece apart from the body parts / texture...but as a result of building the whole construction of the box 'around' the front panel...which would make no sense for me.
That would mean they made the full front panel - Han and the carbonite texture ... then they saw that this panel is not totally square...and because of that they decided to build a box which fits this unsquare panel? Seems irationally to me...