Digital BW, The Print

I bought a Kodak grey scale, to help me calibrate my quadtone curves. I
got a 21-step scale, to match the 21-step stepwedges commonly used in
quadtone work. But the "21-step" Kodak scale only has 20 steps! So:

1. Why is a 20-step scaled called "21-step", and
2. Why 21 steps in the first place?

Harry