User blog comment:Miskos3/Fairy Tail Chapter 490: Fairy Tail Zerø, Review/@comment-192.206.56.1-20160620155411/@comment-9595671-20160620171232

Considering Zeref was on Tenrou Island for a long time, I bet Mavis' astral form may told him about this, and she didn't realize it would be a bad idea until much later.

Kinda like how she cried over choosing Precht, only for him to fall to the dark side and become eventually become Hades when she started talking about it.