That coin has been stained or otherwise environmentally damaged. It could have been buried or covered in cupholder soda leftovers, etc. It got into circulation again, so the handling rubbed the gunk off of the higher surfaces to reveal what you have. It is commonly called "circulation cameo."
Tantum religio potuit suadere malorum. -Lucretius
"Just because you're paranoid don't mean they're not after you." -Kurdt Kobain