Discs are stiffer if the vanes twist, but to cast two parts when one "will do" costs more.
I used to get handed Supra discs, the part number had an L or R after it - but that's rare.
The Merc ones are drilled in a (single) spiral too! So it's in people's faces that they are being tight....but 99% of people wouldn't notice so they obviously decided to save some cost.