I think most people don’t need to worry about that too much anyway, as they most likely don’t qualify for Aspen or Birch (but can possibly choose to switch to that slot).
To my knowledge, it is a common thing to iterate on the accuracy (clearance) of mechanical parts - I remember reading about Tesla not living up to expectations in that regard in early models.
What this most likely doesn’t mean is that the case makes rattling noises or things like that.
If you zoom at this picture, you can see some “imperfection” at the display corners. That would be that.
The batches until Dogwood will have refinements on that case.
I assume, these refinements also serve as a way to iterate before creating the mold of the final case.
Some related info on molds: Librem5 silicone cover
Definitely, Evergreen will be the real deal.
Fitting name 
Supposedly, we’ll soon learn whether we can later get that molded case for previous revisions (which I expect).