My partner and I have talked for a while about this phenomena: there is no "canon" to Arthurian Literature. None. There is a huge body of work, but none of it can be considered canonical.
Lancelot comes about because someone (potentially Eleanor of Aquitaine) commissioned a writer to basically expand the story in a way that she would enjoy. Perlesvaus (The High Book of the Grail) is someone processing their trauma of war, and probably specifically from the Sack of Constantinople.