It's because of how the game populates enemies. When the devs make an enemy group, they choose the levels they spawn at. For instance:
(these level numbers might be inaccurate, but they're for example)
Hellions won't spawn past level 10. No matter how high level you are, you can't fight a Hellion above level 10, even if you're set to +4. A level 6, 7, 8, 9, or 10 hero at +4 won't get a Hellion to spawn past level 10.
Arachnos spawns from levels 1-54. At level 1, you won't be fighting Bane Spiders, but at level 50, you will be. Even at level 50, fighting at +4 works because they're designed to go to 54.
"Old School" Banished Pantheon might only spawn from level 15-30 or so.
"New School" Banished Pantheon will spawn from level 40-54.
Why are there two groups? Probably a technical limitation that doesn't let you go back and change the level ranges of enemies. They solved the problem by making two groups called the Banished Pantheon.
When you're in that "gap", the game will choose the group that's closer to your difficulty setting. Being 31+4, that it most likely rounds up, and gave you the level 40 BP from Dark Astoria.
I had reached level 31 and noticed I had a few Tobias quests left and decided to finish them. the last quest being 27 so I bumped up the level to +4 so it wouldn't a total waste of time. Not sure of the quest but it had old school Banished in it. After finishing that quest I got a new one 'destroy the portal' or something like that which again had Banished but this time they were new school Banished.... AND LEVEL 40!!!!!
I mean really, where else in the game can you be pitted against a +9 while inside a regular mission? I couldn't understand why I was vaporized so quickly by them until a teammate pointed out their level.