User blog comment:Stone Roger/Natsu and Zeref have somekind of relation?/@comment-98.251.3.213-20140406001840/@comment-7418318-20140406081954

In the after words of the volume Mashima wrote that he got the idea of locking Natsu inside the guild hall at the last moment while already drawing the chapter, so I seriously doubt it has anything to do with Natsu's origin or real date of birth.