Fair enough. I didn't consider that.

She was a woman, raised in the rural South during the Great Depression. She likely never went to school and certainly would not have had occasion to write her name, let alone recognize it if anyone else wrote it down. In many parts of the rural South, most people signed their names just by making an "X" in front of a witness, who certified that the person signing "made his mark". Which is where that phrase comes from.