I think
@faatal explained this already if you care to try finding his post about it.
P.S.: Here's what Faatal said:
"Bloodmoon parties (which is now players within 80m of any other player added to the party) now cap the amount of enemies spawned at one time to 30 (subject to change) to improve FPS. An 8 player group with the default setting of 8 per player, would have gotten 64 active enemies. To compensate for less enemies, the game stage increases based on the ratio. The game has global enemy caps and game stage maxAlive caps, but this helps with the specific case of FPS tanking during blood moons for a BM party."