User blog comment:IamJakuhoRaikoben/Chapter 323: The Shadow That Comes And Goes, Review./@comment-4933386-20130316011049/@comment-188.171.27.28-20130316104627

In the timeline that Future Rogue came from, Gajeel probably beat Rogue without any weird power-up for both of them (just like it was before Rogue was possessed). So, FRogue thought he'd be able to win the battle by possessing Rogue, but he didn't except Gajeel to eat the shadows, which I guess never happened in his own timeline.

Most of it is speculation, but FRogue does seem surprised by Gajeel's actions.