I have the min/max enemy level set to default (min 1, max 5) and yet I'm encountering lv 7 biters, and level 9 nests and worms at 68% evolution. Am I missing something or is it a bug (I do have other mods installed, but nothing that affects evolution afaik)?

That's right. Regardless of the setting, Biters will level from 1 to 10. However, their health, attack, and so on will be recalculated. So, with a "maximum ending level" of 5, a Biter at level 10 will have 1,000 health, instead of 30,000 if the "maximum ending level" were 10.

Ah, thanks for clarifying. I wasn't being scammed, the mod is just hell.