Infinite Justice
Member
Before entering a quest, there's a list of fixed rewards that are guaranteed to drop depending on your rank. Sortiara and Avalon bosses give you completely different rewards. Sortiara bosses also feel much easier for me, so I used them as practice to get familiar with their attack patterns.
I'm confident they are easier so the player can be roped in gently.
Plus its like the same concept in Monster hunter where you fight a "weaker" version of the monster in single player (village quests) and the version for multiplayer (guild hall) is "stronger".
So for SS Avalon is pretty much the guild hall (its positioned as such already in the story)