Board Thread:Questions and Answers/@comment-2037710-20130828044130/@comment-6697950-20130828074154

Well, anime subs are most likely the fanmade ones. The original is a novel and there it was clearly stated that "Excalibur" was intentionally changed to "Excaliber". Even the translator who did the translation of the side story involving Caliber made a note:

"The latter was written キャリバー (kyaribaa) which was used throughout the entire SAO series as the strongest sword in ALO as well as this story’s title, the first one was written カリバー (karibaa) just like in King Arthur’s sword Excalibur, even native Japanese could be confused with the two thus what they are discussing in this chapter."