Because only one side is fleshy. The other side is kind of thin.
I think the best fish I ever ate was flounder which we gigged out of Choctawhatchee Bay. My friend coated them with flour instead of corn meal and to my surprise it was better that way. We did deep fry them.
There used to be a place in Pensacola which served a cheap but delicious mullet platter. When they got it just right they were about as good as it gets.