Nearly all lathes display the x position as twice the actual distance from x 0 so you are seeing the diameter of the part. When simulating lathe toolpaths and looking at the "Info" tab I have to mentally double the x position to figure out if the tool is indeed cutting at my intended diameter. Perhaps when simulating turning toolpaths a "diameter" could be added next to the "x position" The displayed number would simply be the x position doubled.