Holes are an issue with all slicing programs available at present. The reason is that the slicing program assumes that the extruder traces a path that is along the exact centre of the extruded plastic line. A circle however has a smaller circumference on the inner edge than the outer edge, and so the plastic is pushed toward the centre of the circle, producing an inaccurate size. While it would be possible to compute the required offset to put the extruder over the centre of the
volume of the extrusion rather than the geometric middle, the problem here is that the slicing program does not know that the head is describing a circle - the STL represents a circle with a series of short lines so it would take more serious analysis to determine the radius of the curve being approximated. The same thing happens on a corner (which can be thought of as a partial circle with zero radius), and any other change of direction, but the distortion is at a single small point and usually absorbed into the body of the print (and the dimensional accuracy is also usually less critical), and so less noticeable.
Dave
