The extrusion width is larger than your nozzle size which can cause unpredictable extrusion. The nozzle is waiting in a spot for the extruder to push out enough material to equal the amount that a .5 nozzle would produce, only through a .4 nozzle size hole. If the feeding of the filament, temperature, isn't perfect the amount extruded will vary and sometimes software over compensates.
You can try playing with the filament size and see if that helps.
I like to use the nozzle size or smaller for the detail it adds. If your needing to print at .5 widths use a .5 nozzle.