Experience, Gold, Runes, leadership, etc. variation

[KongMysen]

I just engaged fights against the Castle on Settera Today. The Samman Bounty.

I fought him twice since my demonololgist' cerberi finished the fight before I could ressurect my casualties. First go gave me around 9000 xp but the 2nd gave me 12000 xp.

I'm not sure why there's such a big difference between these two fights.

A few suggestions:
Summoned troops give xp? (Samman used Demon Portal)
Mana used by the enemy give xp?
Not loosing troops gives a bonus?

I'm guessing the Demon Portals, but I really can't see how those few portals can boost xp like this.
I think finding the correct mechanism require a lot of testing.