And you know that because you are familiar with the evolving legends, and it is providing historical justifications for characters that are certainly latter accretions.
Most "historical" accounts of King Arthur will have Lancelot, and many have Galahad, but we had slews of King Arthur tales from before the existence of those characters, in the days when Sir Gawain was without a question the greatest of King Arthur's knights. Lancelot was clearly developed in response to the refitting of the legends for the purposes of courtly love. And Galahad was clearly developed in the same way for the Holy Grail. We can trace their development in the manuscripts. (And some writers try to evade it by giving Lancelot the name of a knight found in older works. Then why was he renamed, if this is true history?)
Or Robin Hood. With Maid Marian, Friar Tuck, and Alan-a-Dale -- and I sigh. Robin Hood's oldest companions are Little John, Much the Miller's son, and Will Scarlet/Scarlett/Scarlock/Scadlock/Scath
If the writers put them in illo tempore and give them a mythical aura, it might work, but overwhelmingly it is presented as this is what really happened.