User blog comment:FairyRaven/Fairy Tail Chapter 370: Demons Reincarnated, Review and Commentary/@comment-5598380-20140207120330/@comment-7418318-20140208230944

I don't remember when it was ("Next Generation" maybe?), but I think that at some point Makarov said something along the lines of "Three Dragon Slayers in our guild. Maybe Porlyuscia was right." without ever elaborating what Porly might have been right about.

My guess is that Grandeeney told Porlyuscia more than just the spells for Wendy. For some reason Porly kept this secret from Wendy (maybe at Grandeeneys request) but still saw it fit to share with Makarov.