Just doing some numerology on it, if you add up 4 extrusions of .5 mm each, with the two middle ones spaced .1 mm apart, you get exactly 1.6 mm wide, which is the width of that wall. Except it then has to adjust the flow so as not to put down too much plastic. And the two middle paths will still overlap, which seems odd on a single layer.
It works ok whatever the logic is, but the printed walls do have some waviness in them. I wonder if there's room for improvement in the gcode being generated. Of course I wasn't helping things by choosing an extrusion width of .5.