Why does Scout's cousin call Aunt Alexandra grandma? Isn't she his mum?

Cause hes the third generation in the family

Scout is second gen

Francis is Aunt Alexandra's grandson by her son Henry.

"I suppose I should include Uncle Jimmy, Aunt Alexandra’s husband, but as he never spoke a word to me in my life except to say, “Get off the fence,” once, I never saw any reason to take notice of him. Neither did Aunt Alexandra. Long ago, in a burst of friendliness, Aunty and Uncle Jimmy produced a son named

Henry, who left home as soon as was humanly possible, married, and produced

Francis. Henry and his wife deposited Francis at his grandparents’ every Christmas, then pursued their own pleasures."

Thanks!