I thought the exact same thing! Though I guess it only in the last 3 chapters that gets brought up whilst Marth's reval is chapter 6 and Lucina joining is chapter 13, exactly half way through the game. They also wanted to show off the dual support system since that's knew and the avatar and Chrom do have a 'special bond'. Now they'll through in an Robin/Grima staue just to spite us though...