My white dinner plates get grey marks on them if they are scratched with cutlery, why?

It's not the plates, it's the cutlery. A tiny bit of metal comes off the utensil and marks the plate. Same thing happens to porcelain sinks when you put metal utensils in them, and to toilet bowls if you use a toilet brush that is worn enough that the metal interior of the brush scrapes the porcelain. I don't know why it happens to some materials but not others. I've never had it happen to dinner plates.