OP, I always liked the idea that the WoL actually hears the music being played, and that is why their summoning on the first resulted in not only a skewed version of the primal they actually fought, but the music is also similar; but not quite right.

Some of my own head canon revolves around Fray, and the WoL's affiliation with him is not by mere chance, but orchestrated by none other than Fray after he witnessed the rain of blood as the WoL single-handedly obliterated the entire HW guard, and saw nothing but fear and dread in King Thordan before being finished off. It was then that Fray decided that he would surrender his soulstone to the WoL, and willingly lost his trial by combat in Ishgard despite exhausting his competition.
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I know there is no way this could ever be true canon as players get access to DRK well before the Thordan fight, but this is how it happened in my own head because I did not unlock DRK until late into HW.