Haha - regional things I guess. We were on the east side of PA and every kid I knew had at least one set of grandparents they called Mom Mom and Pop Pop. I had never heard that before then.
My wife was very careful to negotiate these things when we had Sarah. Her mom did not want to be a “Nana” so became “gramma.” So the wife’s side was Gramma and Pappy. Mine were Grammie and Pop Pop.