Our ancestors had varying levels of literacy, and many couldn't even write their own names. It's possible that Ann couldn't write, and when she had a situation in which her name needed to be taken or if she had asked someone to write a letter for her, she told the transcriber her name. The transcriber may have had shaky spelling skills, or Ann had an accent that made it sound more like "Comengs" than "Cumming", so that's what they wrote down.