Because you are using the grid on the chart as the index and reading off the heading on the scale of the plotter. Start with a track that is due north. Place the plotter on your departure point with the long edge along your track and the meridian on the chart will be below the 360deg marking on the scale. Now take a track from the same departure point with a heading of 030. Your plotter will be angled 30deg to the right of the meridian. The same meridian will be below the 030deg marking on the plotter. To be able to do that, the 030 marking on the scale needed to be 30 degrees to the left of the 360 marking.