x does not mutate at all. x is simply a placeholder for the values in the series of additions that your expression expands to, that is, 1² + 2² + 3² + ... + 10². Or stepwise, to 1² + sum_{x=2}^{10}x² etc. all three expressions are equivalent, and no mutation of x occurs anywhere

The series of additions can also be infinite, in which case it would be quite difficult to expand fully, but that doesn't prevent you from doing mathematics with it.