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I read an interesting article from the OMRS Journal, September 1968, authored by W P Dawson.

His explanation for why QSAs have ghost dates is that the crystalline structure of metal is altered when it is subjected to pressure, the degree of alteration being proportional to the pressure applied. In removing the dates, the metal below the dates is subject to a lower lever of pressure than did the surface of the medal when it was struck. As a result, the structure of the two areas of metal is slightly different. As the medal oxidises, the two areas react differently to air and result in the appearance of ghost dates.

It is a most attractive feature and something that I have always liked to see.