I happened to see this mentioned somewhere presumably as a joke, but my nan insisted on being called Grandma, as she said nan was a goat etc etc.
I hadn't heard the above reason until my uncle mentioned it a while back. Also presumably she thought it common.
I always thought it was sooo formal - quasi-Victorian, although she will've been born in about 1915 or something. She died in the early 90s. I don't recall ever challenging the status quo.
She wasn't well off financially or anything.
Anyone else?