Actually, it's 1024 bits, but you include ZERO, so the range is 0-1023.

But your math is always divided by the MAXIMUM (Full Scale) value (which is 1023).

That is because there are 1023 equal portions of whatever you are dividing up to make your whole. Zero is not a portion! I'd get the hump (big-time) if I went into Maison Alain (French Restaurant) and got the Zero portion of the Cream Cake!