This is all spitballing here, so don't take what I have to say too seriously.

One possibility is that the Pixies underwent a radical transformation on the Source as a result of one or more calamities. It's possible that one or more calamities resulted in Source Pixie souls being too dense to remain as Pixies, and they took on new forms instead. Another is that Pixies themselves are the result of an emergent species. Perhaps every Reflection has Pixies and the Source never did, because of some quirk involving the Reflections' connection to the Aetherial Sea.

Maybe the Pixies on the Source are just hiding. Maybe the WoL is a Pixie in disguise.