User blog comment:FairyRaven/Fairy Tail Chapter 329: Seven Dragons, Review and Commentary/@comment-49.176.34.76-20130430152649/@comment-5840892-20130430160249

My understanding about the Key to the Starry Sky Arc was that Mashima wrote himself for them to use as an Anime only Arc, but was originally going to be used in the manga. It happens sometimes in One Piece; Oda has a story he wants to do, but due to time and plot timing constraints he can't fit it in. I was under the impression that Mashima wanted to use it as a Manga Arc, but due to the anime catching up to quickly, he gave it to them (The Anime Production Team) for use as a Anime-Only Arc; so that he could get farther ahead in the manga. I personally see it as cannon, but that's just my opinion.