Funny thing that I did as a child and not sure why, or even if I noticed I did it - on my mothers side my grandfathers were Papa (My mothers father, my mothers step father and my great grandfather). While on my Dad's side, it was Grandpa. All my grandmothers were just "Gran" though.
My aunts name is Janet, but I guess I found that hard to say as a kid, so I call her "Aunt Net" to this day.
Weirdly, my little cousin calls my father "Uncle Piko". His name is Fraser. WE HAVE NO IDEA WHERE THAT CAME FROM.