User blog comment:Omega natsu2/Chapter 354: Ephialtes, Predictions./@comment-8153438-20130927123807/@comment-7418318-20130927130417

As FTGM said, it's not about what we know now, it's about what the Council thought back at that point of time.

And back then they thought Siegrain was a real person and a fellow council member.

The council first hear the name Jellal when "Siegrain" convinced them to fire Etherion and told them that Jellal was his evil twin who was trying to revive Zeref.