inconsistent parameters passed to onMove of moveable point and line segment

Can someone tell me if this is a bug in the interactive "graphie" code, or just in my understanding of how it is supposed to work? The x,y values passed to the onMove method of a movablePoint are in absolute coordinates of the graphie window. In contrast, the x,y values passed to the onMove method of a movableLineSegment are deltas, relative to the previous mouse position. (In the exercise: use mouse to move the point and the line segment. Parameters to onMove method are shown in labels at top and bottom.)