Good question!
In KISSlicer, the nominal extrusion width w is interpreted as if the cross section of the extrusion was rectangular. Let h be the layer height, then the area of the rectangular cross-section is A=w·h, and KISSlicer sets the current extrusion rate accordingly. In reality, however, the sides of the extrusion of a peripheral loop are not rectangular but round (like a smaller rectangle flanked by two half circles left and right), and since A and h are fixed, the true shape turns out to be a bit wider than the defined extrusion width w. How much wider depends on the layer height h. The width of the rounded shape is w+(1-pi/4)·h, so assuming w=0.3 and h=0.2 the thin wall should measure w'=0.343.
By the way, when calculating a correction factor for the extrusion gain, the measured width should be corrected for the pi-term. Say the extrusion gain e results in the measured extrusion with w' of the thin wall. Then the corrected gain e' is e'=e·w/(w'-(1-pi/4)·h).
Side note 1: The effective extrusion volume depends somewhat (very little though) on the back-pressure caused by the material flow in the hot end and the printed part itself. For example, when printing the outer loop (or a thin wall), the extruded material has more options to evade than, say, when printing the inner loop where the material has to push against the next outer loop. I still would not recommend to set the extrusion gain so that the thin wall object will be over-extruded, but it should also not turn out to be much under-extruded.
Side note 2: Other slicers might interpret extrusion width and extrusion correction settings differently. SFACT, for example, interprets extrusion width as the width of the thin wall (i.e., extrusion width is taken as the width of the rounded extrusion shape). Moreover, the correction factor for extrusion is the inverse of the extrusion gain.