I think that in the original control that you downloaded, the buttons were labeled a,b, and c but the actual numeric value of the slider was 2,1,0 - the bottom of the slider is 0. So if you've selected the "a" button, it has a value of 2. The text labels are completely abritrary and probably should have been something completely different to avoid confusion. You can always use the probe tool, wire in a normal numeric indicator, or turn on digital display to check a slide with text labels.