Name a few, and let me know how they're different from one another in as dramatic a way as the G43s are.
PTRS.
Guards one actually hits and kills infantry.
USF carbine vs elite carbine.
Soviet guard mosin is actually good on the move as well as long range, while con mosin is mid/close range primarily(yes, you can use it at long range too, but we're talking scenarios, where you wouldn't lose more then gain from engagement).
Weapon crews have completely different curve then their mainline equivalents.
LMGs behave very different when used on certain elite infantries(either massive performance boost, as with ober 34 or firing on the move which is not possible for regular infantry).
There is also BOYS AT rifle which behaves completely different on sniper and completely different on AT tommy section.
That would be couple of examples of the top of my head.
Flamethrowers behave completely different depending if infantry uses them or vehicle.
I'm not sure why you think pointless inconsistency and lack of explanation by the game is fine because "You should just know it".
Just because you don't see the point, doesn't mean its pointless. It simply means you lack the capacity to grasp broader picture.
But don't worry, this is exactly why forums exist, if you ever feel confused again, post a thread in strategy section and someone will explain to you how things work and you will no longer be confused.
Do you honestly think that most players just inherently "know" all of these inconsistent quirks? People on this very forum are constantly mistaken as to how the G43 in particular even acts.
I think most players who actually use them in game quickly get the grasp of what the weapon can do, you'd have to be exceptionally smooth brain nugget to not tell the difference between gren G43 and sniper G43, I mean sure, there are people who can't tell, but game isn't exactly balanced with intellectual amoebas in mind.
Give me one good reason for any of this to be the case.
Different troops with different purpose on the battlefield used same weapons.
Not every unique unit got snowflake fallshirmjeager treatment in ww2, where they got unique snowflake weapon designed just for them.
All, front line grenadiers, cannon fodder volksgrenadiers and snipers used both Kar98s and G43s.
So not only it makes complete sense why different units would use same weapons but use them in different ways, its hostoricaly accurate as a cherry on top.