I grew up in the north. My mums parents were grandma and grandad. My Dads mum we called Nan, though this was an agreement between my parents and them i think. More to avoid confusion. My Nans second husband (my dads dad died before I was born) was Grandad Alan. I haven't really ever equated a difference, my Nan's other grandkids (on Alan's side) call her Grandma or Granny.
Though I doubt that particularly helps. They generally means the same thing, though in higher social circles, Nan or Nanna (from Nanny) refered to the person looking after them or so I am led to believe.