Somehow I doubt the Script actually referred her as a southern Earth Pony

@FlonnesIkramant  
oooorrr they did.  
it is alreayd implied that both meadowbrook and mistmane, an eastern unicorn, are connected. and both are ponies who were known for healing. could be myths and legends were mixed.

It isn’t addressed either. Most likely explanation is they just didn’t think about it when they made up her design.

Oh my god it’s My Little Pony not Game of Thrones

Yeah, weird, isn’t it?