That's interesting. The only thing regarding the mirror that I can think about, between RHD and LHD, would the positioning on the door. Not sure why the mirror itself might be different. Maybe some forum searches are in order.
They made a change a few years ago, making the
availability of convex glass (OBJECTS IN MIRROR ARE CLOSER THAN THEY APPEAR) for the RH mirror of a LHD car. That's pretty standard on all cars today. Since the mirror glass between LHD and RHD is identical, one can use this convex glass (I have) in both L and R mirrors. I think it improves overall vision in the RH mirror particularly, but also the LH.
MAYBE that's what they mean.