That's a bad choice of name from one or more old languages that misused established mathematical terminology (e.g. C misuses void, constant and variable). An immutable variable (or in maths, just "variable') has a value that is unknown at build time but is constant at runtime. A constant has a known value at build time.

What nearly every language calls a variable today, is probably better described as a "reference" or "assignable", but sadly that ship has sailed and we'll forever be inconsistent with the math terminology for that one. Immutable variable is therefore a useful synonym for "proper variable".