This may be a feature of the game, but trained units gain XP 50% as fast as champion units.
Is this intended? Seems like a bug to me.
First pic is my Sovereign, Kul-al-Kulan. He's gained 27 total XP so far (21 for lvl1 + 6 in lvl2). Second pick is a trained unit, 13 XP. They have both fought in the only two battles of the game to this point.

