Mutability is distinct from variability. In Javascript only because it's a pretty widely known syntax:

    const f = (x) => {
      const y = x + 1;
      return y + 1;
    }

y is an immutable variable. In f(3), y is 4, and in f(7), y is 8.

I've only glanced at this Zen-C thing but I presume it's the same story.