Not sure about this, but the machine that paints the line probably has its steering/turning circle set to the required radius/diameter/angle and then just drives in a circle, same as if you held your steering wheel just off centre, eventually you’d end up back where you started. If it’s a few centimetres off, it probably doesn’t matter.

Either that or it just follows the edge of the thing it’s drawing a circle around

So it’ll just be the border library then?