Ground is never a step; zero or one; not. Because while we are on ground; we are on same level, stairs have not started yet. As soon as we put our foot on upper level than the other foot, stairs have started & that upper foot marks the / lands on "first" step.

Following the principle that array indexes are “between” the elements, the bottom of the first step is 0, and the top of the first step is 1. Then the ground, which is equal in elevation to the bottom of the first step, is at 0, and the floor after the last step is N, one past the index of the last valid step, N-1.

Half open zero based ranges are quite satisfying :-)

I think it's the same people who call the ground floor the first floor. Coincidentally, I often have an off-by-one error in elevators.