User blog comment:IamJakuhoRaikoben/Chapter 464: Natsu vs Zeref, Review./@comment-26451407-20151201010113/@comment-7418318-20151201195358

Apart from what the others said, Zeref was clearly caught off guard by Natsu's sudden powerup compared to the last time they met.

And he didn't exactly seem to be going all-out either.