[aldarion]

Tolkien was not a medieval historian himself, but he was the next best thing to it: he was a professional linguist. And lingistics require knowing and understanding historical context in which languages had developed. Add to this Tolkien's own experience in the First World War, and it is not very surprising that his world is very realistic and internally coherent.

GRRM was... a script writer. He manages to get some of the details correct, but once you scratch below the surface, his worldbuilding largely falls apart.