The other factor involved in the zone system is development
: by over- or under-developing the film, the contrast is changed, which again influences the final gamut
of the image.
Notice, though, that this is practical only with large format, where film comes in sheets that you can individually develop. A spot meter is also handy.
Ansel Adams actually invented the Zone System: he always stressed the importance of previsualization for the photographer.
The Zone System (and Adams' photography) is about absolute image control. You are master of what ends up on paper.
It also worth observing that the Zone System in all its might only really works for Black and White, where you can pretty much play around with development times and fluids.
In color, you have much less freedom with the negative: you can still modify things when printing, though.