They are not a separate subspecies. They are already in the gene pool of the common carp, so a certain (low) percentage of the carp hatch each year turn out to be leather carp and mirror carp. It's like a red-haired kid will pop up once in a while in a family where neither parent has red hair. (No jokes about red-haired mailmen, please
The percentage seems to be higher in Europe than in the USA. I was looking at a German encyclopedia and came across a fish picture labeled "Das Karpf" (The Carp) and sure enough, the fish pictured was a mirror carp.