If you leader is battling with other champions that can reduce the XP your sovereign gets and thus you have a lower level. Note that you get reduced XP only with other champions in the party not the other units.

The AI uses Autobattle which in some cases is better off than using manual battle, in other cases it is worse off. Without experimentation its hard to say if there is an advantage to using it but that could be the case. The benefit is loosing less hp/battle letting you fight without having to rest.

Another thing is that the AI isn't afraid to suicide their sovereign and champions against monsters. This means that potentially they can level up faster by fighting battles that they can just barely win while human players might wait until they are sure they can win. 

I know personally my leveling is slowed because I like to have my heroes trade items which the AI probably never does. I also focus on picking up treasures lying on the ground more than killing monsters, while the AI sometimes ignores treasures and focuses on monsters. Over the course of 100 turns these small advantages can add up. Also, now the AI seems to get potential pretty often so not choosing potential could slow you down as well. Don't forget about the 10% bonus for Altar and the 10% bonus for having the Brilliant trait. 

Its not the number of battles you fight, its what you fight.

Tougher monsters give tons more exp than weaker ones. The cignys (sp?) for example gives tons of exp. Kill 3 of them in one fight to get what you get from 10 mite battles.

Thanks for the answers!  My Sovereign has been in a party with 2 other Champions, so the XP has been shared.  I'll have to split them up if I want to level them faster.