The monsters can be different levels than we are.
As for why Rengol elites are lower than Trindin, I believe it is because the "base" power of them is lower. So, a level 1 Rengol mob might have ten more damage than a Trindin mob. Scaling the mobs stats both up to around end game levels, the Rengol mobs are probably much stronger when at similar levels. Thus, a higher level Trindin mob may feel weaker than a Rengol mob.
If that doesn't make sense, think of Rengol mobs as mythic and Trindin mobs as legendary. A level 36 mythic will be better than level 37 legendary.
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