User blog comment:LordLuminous/Silver being Deliora = Impossible? Theory/@comment-24734283-20140705195126/@comment-85.221.190.231-20140706080051

IIRC he only thinks "Natsu of all people" after Lisanna called Natsu by his name - he didn't recognized his face. We know that Zeref knows Natsu. Perhaps demons studying Zeref heard about this relationship somewhere? Pure speculation, but maybe there's a prophecy that Natsu will kill Zeref, or E.N.D., and Tartarus knows about it.