These enemies are stronger because some inherently have higher resistances to certain things. Like mummies might have no fire resist or even negative fire resist, whereas skeletons might have higher resistance to nature damage. Not saying this is the case, but its an example. As you face harder enemies, they have higher resistances innately. Also, definitely hold shift to look at their equipment. That way you can see if they are wearing something "of the tempest" for extra electric resist etc.