My only guess is that at one point after taking it from the stone, Excalibur breaks and Arthur has to take it to Nimue to have it fixed.
That's not an uncommon plot thread in Arthurian depictions. Both Tennyson and White's versions of Arthur breaks the Sword in the Stone in a clumsy and prideful duel against Lord Pellinore, and goes seeking Excalibur to replace it.
In the 1981 movie,
Excalibur, they're both the same sword, but Arthur proves himself unworthy by striking down Lancelot in a dishonorable fashion, and the sword shatters on impact. He regrets his mistake immediately, and is about to throw both it and his kingship into the lake they're fighting in, because of how disraught he is about breaking the code of chivalry. Nimue is so impressed by Arthur's humility that she surfaces through the lake, and repairs Excalibur for him by magic.
Bottom line, Arthur's sword breaking is a big narrative moment for him as a character...which is why it boggles me that it would be in this
first film, during Arthur's youth. The whole point of him breaking the sword and becoming unworthy is that it happens years later when he's an older man, and should be above youthful impulses.
But, hey, what's one more tainting of classic Arthurian motifs, right?