User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-5303512-20160927112257/@comment-5303512-20160927121845

See, I thought that too, that when Zeref pierced that book it would leave a "void" in Natsu, but that wasn't the case, it seems that spot became of sort of bridge for whatever was inside that book to become part of Natsu.