The XP from the 50 dungeons could be set to scale from Aurum Vale up to Dusk Vigil. I'm not sure how they even come up with the XP per mob, but to me it'd be quite simple. Come up with an XP per run rate, divide 40% among the bosses so that the first two give 10% while the last gives 20%, then the other 60 is divided between each of the mobs in the dungeon. If the mobs are optional such as the eggs in Aurum Vale then it can break this limit with them hatching, but those are few and far between enemies. Doing this however would make it easy to scale them between AV & DV, whilst also creating a fairly simplistic streamlining process in which to add XP to the old dungeons. Even if they only ever used such a formula for that purpose, it'd be simple and effective.