You don't have to
Items of quality have specific mythical qualities, not a generic betterness. The Verditious magically brings forth the natural correspondences with the realm of forms. Not a generic +3.
You seem to be confusing excalibur with the grail.. excalibur doesn't have much to do with the divine at all in the various tales, only the grail does.
Excalibur would either likely be made from the magic realm, or alternately by the faerie realm, depending which particular tales you preferred.
The sword in the stone was probably magic, as it might well have been made by merlin, but the sword given to him by the lady of the lake was likely fae enchanted.
I think I m quite capable of distinguishing between the Grail and Excalibur thanks all the same. In point of fact the legends of Arthur and Excalibur as proliferated widely in the context of mythical Grail Christianity (that is Mystic Christian beliefs), all that you seem to take as literal based on surface readings of the texts are in fact largely allegories or symbols of theological teaching.
Insofar as Excalibur is exemplified as possessing powers of light and truth and further insofar as it is the signifier of the Divine right of Arthur to rule it could be just as viably argued that its ultimate source of its power is of the Divine and that either the Stone or Lady of the Lake motifs each carry their own allegorical significance to the Grail Christianity ethos.
I have read that the stone/anvil scenario is taken by many to signify to the extraction of the pure spirit (Excalibur, the sword of truth/light) from the base nature of man, as one but one example of this argument.
Just as the above demonstrates that one could deem it a device of Divine Realm origin.
If one adopts pure literalist readings of the text, sure. If not there is a third alternative.