Wow, nothing, not even a quip saying that's what the Zone System does.
I guess I haven't gotten my idea across or nobody is reading the book.
Basically the idea is that an "S" shaped curve properly matched to the paper will put more zones on the paper than a strait line curve of similar density and straight line steepness might, albeit with some compression.
Ctien goes on to say that his preference is for long straight line curves because of his subjects, good separation of tones at the ends seems important to him.
That though is his subject/style choice, may or may not be what you or I want.
For short scale primary subjects, maybe a portrait or a flower, in a long scale setting, we may want to purposefully sacrifice some of the setting's "quality" to keep the viewer's attention on the main subject.
Another example is TXP's curve. Zone for zone, the long toe on TXP should print more zones below middle than than TMY. Sure the separation falls off too but that may actually be what we want on paper.