User blog comment:IamJakuhoRaikoben/Chapter 327: My Part Too, Review./@comment-5113042-20130413152152/@comment-15267891-20130414005743

theory: Zeref was the one who sent the dragon slayers to the future and he personally knows Natsu at that time, when Natsu is still a kid. Remember on Tenroujima, when Zeref saw Natsu, he said: "Natsu, you've grown"