User blog comment:Omega natsu2/Chapter 400: Wings of Hope, Predictions./@comment-3994996-20140911221724/@comment-7418318-20140912075845

Well... the one year was in a different future were no Dragons at all came through the portal. The seven Dragons and the wide-spread destruction they wreaked in the capitol city might have sped up Tartaros' plans and/or Arcnologia's appearance.

But back to the original post: Natsu already knew that Acnologia wiped out the majority of mankind in F-Rogue's timeline. Would it really be surprising to Natsu to hear that apart from millions of Humans, Acnologia also killed Frosch?