That's my understanding also. Its from the excess carbon nodes interacting with the surrounding iron. Doesn't happen to any great extent with steel, but I don't know for sure but the high silicon aluminum has excess silicon crystals which may react in a manner similar to the carbon crystals in cast iron. A good engine of any material will be heat treated to prevent this. It was one of the differences in the old days of cast iron pistons between the likes of Enya and Fox vs say a McCoy.

The piston skirt can wear because that is the primary part of the piston that contacts the cylinder. That skirt is there to prevent the piston from cocking inside the cylinder. If worn excessively you can have piston slap which will in turn wear the ring groves. A good overhauler will check for wear on the piston skirts, some even check the diameter at the skirt.