User blog comment:817603/Summoning the Celestial Spirit King/@comment-26572767-20151207025308

Well Someone told Lucy that the 13th key was a myth. And tho its apart of the Zodiac, it was a black key.

Maybe Layla broke the 13th key to summon CSK, but then told Lucy there was no key but someone like Zeref/Phret took the fragments and made a new key (black version). Yukino/Sorano were both children that were targeted by TOH people so there is that connection.