Over time, the two stories might have been mixed together and there was no distinction between the two swords left. Arthur might have actually had 2 swords, Amorachius from the stone and the other one from one of the queens, though I am not sure about the order. In turn that would mean, that he was a Knight of the Cross and a Knight of one of the courts of the shide at one point or another.
I am not all that familiar with the whole saga, but didn't Arthur at one point lose Excalibur (Amorachius in this case)? If that is the case, the shide queen in question might have seen her opportunity to take a mortal hero for her own gain. I expect Mab, it seems pretty wintery to me. Though the lady in the lake would be more appropriate to be summer.