Actually, the Runelord party was supposed to be a band of enemies (in your case sun worshippers, in my case troll scum
).
So the lower 'level' party members take on the weaker members of the enemy party.
If everyone in the characters party is 'Rune Level' a balanced foe is going to be largely all Rune Level as well so everyone has a 'fair' opponent. It all seems very contrived to me, and detracts from the realism.
Not every fight is fair, nor should it be. That is one reason I've liked the lethality of BRP. A big part of surving is knowing when to fight and when not to.