may be they are using low quality membrane materials... that doesn't prove every Japanese dynamic headphone is grainy as you claim.
I agree, it does not prove it imo, many are grainy but by no means are they all grainy. Most cheaper headphones in general are quite grainy. Comparing numerous equivalent Japanese phones to others will give a better idea if they are actually more grainy or not, I've personally haven't listened to many mid-tier Japanese headphones, mainly just lower-end or high-end. The most grain-free dynamic I've ever heard is a Japanese headphone. The MA900 seemed to be a bit different than normal in terms of grain, it's like it shouldn't of been so grainy yet it was, I'm beginning to think it could be the impedance resistor that was bothering me.