That's a good question about the layerheight. I have the feeling that with 0.1 mm, the molten filament is squeezed a little bit to the sides because of the flat nozzle. And with 0.2 mm, the filament has more space under the nozzle so it stays in it's "round" form. Putting a string of really 0.4 mm on your print instead of a squeezed string of >0.4 mm wide.
But this needs to be checked.