Considering "The Matter Of Britain" was probably imported from Wales to
Brittany (both Celtic, thank you very much) and extensively revamped by
Eleanor of Aquitaine's daughter by King Louis, Marie de France to fit in with
Eleanor's Corts of Love, then re-imprted to England when Eleanor married
Henry II, it is most possible that the legends have picked up a lot of
nuances from European Celtic traditions in addition to their original origins
in the Mabinogian.