Sforno on Leviticus 13:19/ספורנו על ויקרא י״ג:י״ט

1והיה במקום השחין שאת לבנה, this spot is not being judged by the same criteria as listed earlier which were inflammations of skin on the flesh; similarly, inflammations caused by fire burning the skin. When ordinary שחין destroys the tissue on top of the skin it never grows back to look as it did originally. What grows on the afflicted area is tissue which resembles the original skin, so that no skin graft is needed. Original skin is medically irreplaceable. [opinions in those days.] When someone suffers from a נגע צרעת the skin, when healed, looks indistinguishable from the original.