I think most do this when heated in normal driving and probably worse when abused/heated more. It might be where the spring pressure is applied to the back of the cast friction material and normal heating during slipping. I've found cupping on pressure plates that had almost zero abuse so it's hard to say. Almost every clutch (even stock) I have looked at in my time had more wear on the outer part of the disk than the inside. To me it seems worse on pressure plates that have a higher or cranked up plate load. This will make a puck disk really hard to drive, but organic type disks tend to wear in faster and will have more contact with the pressure plate even when cupped like the picture above. I know when I put in a street disk in my 2600 (same plate) it felt great even after having a brutel experiance with the 6 puck.