I think it's simply because of the randomization. I had the luck of finding (on impossible) dragons on rusty island (tavern), black knights in the shipwreck on rusty, cyclopses and demonologists on bolo and so on. Maybe you just "lucked out" with poorer unit setup.
Maybe when you open up Elon, Dersu, Uzala, Nameless and Reha things will change.
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