It might be that they are made in different factories and even by different suppliers to Shimano. Perhaps as simple as building new tooling and dies to make the part would be more expensive than just modifying tooling and dies that one factory already had. Maybe the plastic used isn't the same for some reason.

It's not really the show stopper for whether I can ride my bike or not. And certainly not for whether I use Shimano cranks or not.

Sure there is a reason why they did this. There are better mysteries in the world to puzzle on though.