Thanks @gr5 !
The tangential nature of a fillet generates the long step on the first layer that isn't that effective because the next layer steps a long way back from it so you don't really get the benefit. Using a chamfer adds the structure more uniformly.
I agree that probably a kind of parabolic shape could be even more optimum, but CAD software is set up to put in fillets and chamfers in a single operation on corners whereas using a non-uniform shape is a much more involved workflow.