Number of monster types per area

[lfd]

I was under the impression that there would be three monster types per area.  However, a recent trip through the Glacial Trail of Hell yielded only Bone Mages and Bone Warriors, and no other monster type at all (well, except for the minions of Bonesaw Breaker).
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(begin handwaving)

I guess that when the game adds the monsters to the map, it randomly picks what type they're going to be from its chosen "possible monsters" list; therefore, there's nothing to stop it accidentally failing to pick one of the types purely by chance. I've certainly had maps before where there have been only one or two or a given monster type.

(end handwaving)

You can get more than three non-boss/champion types if a monster type always spawns with monsters of a different type (e.g. oblivion knights/abyss knights), or if they're built into the map.