To be less cheeky about it, there's a lot more truth, but for the POV charater, it's already 300 years later. So theres been a decent bit of mythologizing already.
That's mostly because i don't want to pin down every element of the arthurian mythos; some of its too hard, and some of it is too cool to leave out.
Think of it like the genealogical stories of the anglo-saxons, or the Danes/Norwegians/Swedes.