Math and physics are precise, art and language are not.
I've actually tried to reconcile that in my head and the explanation that fits is that in a standard contrast scene that spacing is true. The imprecision of language makes this idea a challenge to communicate.
If we use +2 development (to keep the math simple) it's only a 5-stop range (not 7) and zone V is not 4-stops up from zone I, it's only 3 stops up measured using a light meter. In this case Zone VIII is only 2-stops up from zone 5, not 3.