The various curves I've drawn simply show what varying camera settings might do and the problems they might pose. The "under" curve shows that shadow detail on the negative wont reach down to zone III in the print, the "over" curve won't reach up to zone zone VIII. The other two can get us nearly identical prints.
Part of what I'm trying to illustrate is that zones tie the scene to the print, not to the negative. Where our chosen zones fall on film is a variable, not an absolute.