One possibility is just lack of reference photos, even though this seems rather unlikely or somewhat absurd.
E.G. sure you can always visit some museums and snatch some photos, but if there's something missing they might need to find original photos and such to cover it up and it may cause these cross-overs. Kinda like it is hard to model soldier's backside if you only can see a reference photo from the front, unless you know 100% how the back should look.
And another possibility is the fact that there is always some kind of line generalising and details. For the same reason we can presume that as RO Panther looks like Panther and generally acts as one, it works fine overall. Sure, if TWI would want they could have added other models, but I believe they presumed it to be fine and work on the general level.
E.G. german soldier model. To say it roughly every german soldier model would indicate that the guy is a rifleman in the eastern front @ 1941 - 1942, if you just look the details. And then when you pay attension you see that every soldier is kinda like rifleman (there is one thing in german player model which has something that can be considered 'rifleman exclusive', as from searching more or less original photos from books or such I have only seen one photo where guy with an SMG carrying this item). While it is wrong, and especially if we go to details in late war these models are just plain wrong more or less authencity or accuracy wise, they still look good and do their job - to look like german WW2 soldiers. Not to mention soviet player models or other things like that.
So, one big problem, atleast I presume to be, is weenie factor or perfectionism. While yes, you can always note some mistakes (like that typo in the Tiger tank sights) and possibly report them, their gameplay affection value is rather minimal. Expect for those who considers a minor typo in the sights as a major problem. I just don't get it how come someone could make such a fuss and cuss that tiger sights have a typo... when only thing it actually affects is the fact it may look silly. And even if there would be real, 100% original typoed tiger optics, it still would've been in service no matter of the typo.
Sure, I agree that if something is wrong - it is indeed wrong, farby, annoying, violating authencity and such. However, one very good thing to remember is that after all - RO is a game. Not a time machine, not a war simulation. Just bit more realism-biased FPS game. What it is designed to be, generally authentic WW2 FPS game, it works quite fine in that job.