Game! - Where the stick is mightier than the sword!

Blue dragons and living stalactites are about 500. Yet the BD's are significantly deadlier and the LS's.

How are the base XP values set? What makes the LS's worth as much as the BD's?

Looking at the stats for each, they shouldn't actually be that different. The LS hits harder, but has less defense and HP.

But, to answer your question, they're assigned, sometimes more haphazardly than others.