User blog comment:IamJakuhoRaikoben/Chapter 470: The Hybrid Theory, Review./@comment-27674224-20160126181825/@comment-4829046-20160130122509

Wait, the point is that Natsu may have been considered dead? I don't think whatever coma he is in made his scent change to that of a near-rotting DS(whatever that is supposed to smell like) unless Acno is being updated by Zeref and bases the number around what Zeref has said.(don't think so)

A more plausible theory for Invel could be that Acno senses the actual magic and decided the number based on the people that are releasing DS magic around the continent. Zeref may have purposely sent out Serena(who wasn't even an original pawn) there since he is very flashy due to possessing 8 lacrima and would be top priority for eliminating. Meanwhile, Invel could use some forbidden magic to hide his DS power and still provide Zeref with firepower to neutralize some S-ranks while Acno is on the rampage elsewhere.

^Not bloody likely, but just putting out a possible path. Very against them having yet another DS vs. God demon slayer scenario though.