Having the correct weight and correct size clearly implies it is not a wrong planchet.
The rounded rims imply it was bezelled or spooned to some extent, perhaps for inserting into jewellery.
I would tend to assume environmental damage: the coin was sitting glued or otherwise attached to a surface for many years, causing the different corrosion rates on the two sides.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis