'Variable' is a jargon-y term in mathematics/programming in general, it doesn't literally mean "this thing can vary in the run of this program", e.g. even Haske

'Variable' is a jargon-y term in mathematics/programming in general, it doesn't literally mean "this thing can vary in the run of this program", e.g. even Haskell calls its name bindings "variables", and it is focused on immutability much more than Rust (I suppose this is the functional programming aspect you were mentioning). One way to view it is that the bindings don't have a value that is fixed at compile time: they vary between runs. In any case, I'm not sure what you mean by "mutability varies".