This is due to the segmented nature of arcs in SU. When you offset a SU arc, the result is strictly speaking no longer an arc, because the endpoints of all the segments are no longer on the same arc, as the start and end segments of an offset arch are shorter. Arcs and circles in SU are determined by the vertex points of the segments, whereas the offset command is offsetting the segment lines. Note too that if you offset a circle by, say, 1 meter, the distance between corresponding points on the two circles will be more than 1 m, whereas the parallel distance between two corresponding segments will be 1 m...
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