Well, given the scope of Westeros, it’s harder to say – we don’t know how compact or contiguous the lesser Houses’ lands are.
But yes, it’s not very historically accurate for borders to be that cleanly defined, but on the other hand, if GRRM had gone the full Holy Roman Empire for his books, I don’t think people would like that. Way too complicated for new readers to pick up, and probably would distract from plot, theme, and character.