It's a term of phrase ,there is no specific reason.
I think they call her Nana.
Canadians typically call their grandma "Grandma" or "Nana," similar to the terms used in many other English-speaking countries.
Nana it is Jewish for Grandmother
Some names i have heard for grandma is: Granny Mama Mema Mammie Nana
Gran, Granny, Nan, Nana, Nanny, Grandma ...or their name :)
The check word for grandma is "nana."
A British name for grandma is "Nan" or "Nana."
nona,grandma,
nana granny momma oma memaw
Nana was a Naiad Nymph of the Phrgyian River Saggarios.
In American Sign Language (ASL), you can sign "Nana" by using "MOM" with an "N" handshape, moving it in a circular motion on the side of your chin. This sign differentiates "Nana" from "Grandma" but still conveys the same meaning.
nona,grandma,