Environmental Damage. I metal detect and have dug many nickels such as yours out of the ground. Also.. a Cent Planchet is smaller than a Nickel Planchet so this can't be an off-metal error issue here.
Here is a sample of a Nickel struck on Cent Planchet that I own.. It's smaller because a Cent Planchet can fit into a Nickel chamber not the other way around.
When I was about six years old I was tossing a nickel up in the air and catching it in my mouth. Oops. Swallowed it. The x-ray showed it in my stomach, so my parents let nature take its course. Long story short, it had what might be called "natural artificial toning" when it was retrieved. Wish I had that nickel today! Steve