Why is the diaphragm spring matte finish for some of the EXEDY multi plate clutches?

This is because there is a special coating on the surface of the diaphragm spring.
Some of the twin clutches are using two diaphragm springs, one on top of the other.
Operation load (release load = force on the pedal) becomes greater due to the sliding resistance occurring when the two diaphragm springs rub each other during the operation of the diaphragm spring.
EXEDY has reduced the force on the pedal by adopting this special coating.