The Arthurian Legends are a collection of various stories that are all different interpretations by different writers. The Sword in the Stone was Excalibur as it was the King's Sword, a story meant to explain how Arthur came to rule the kingdom. Another interpretation is that he acquired the sword from the Lady and that Merlin was more directly involved in such events. The problem is there you have multiple tellings of the same story from different perspectives. There is the British notion of how things went, the Welsh, Scottish and Irish interpretations as well. Just like there is the comparison made between Merlin and Gwydion of the Tuatha De Dannon.