I just always ended up with it sounding like gramma when I was young. Then it became habit. Now, my paternal grandmother I called gramma (last name), but rarely talked to her, hence using the last name to distinguish her from the nice grandmother! (All my grandparents died when I was young, except for my dad's father who lasted until I was in college.)
Ha! My grandmothers both get EXTREMELY upset if we call them granny! Only my sister ever does nowadays (always as a joke) and she still ends up getting chased with a wooden spoon
I can see where "granny" is a little older sounding than some of the other options, so I'm not too surprised they objected to that particular moniker. And the 'ole wooden spoon thwack, thought that disappeared, ha ha!