I don't know if you're referring to a particular language's peculiarities, but it doesn't really matter. It's mutation.

tombert's point (I think) is that in a functional setting, factorial(n) is always factorial(n). In an imperative setting, first it's 1, then 2, then 6, then 24, etc.

Here's factorial calculated imperatively in C#.

  public async Task Factorial() {
      long primitiveResult = 1L;
      Wrapped<long> objectResult = new Wrapped<long>(1L);

      var observer = new Task(ObserveVariable);
      observer.Start();

      for (var i = 2; i <= 15; i++) {
        primitiveResult *= i;
        objectResult = new Wrapped<long>(objectResult.Value * i);
        await Task.Delay(100);
      }

      observer.Wait();
      return;

      void ObserveVariable() {
        while (primitiveResult != 1307674368000 || objectResult.Value != 1307674368000) {
          Thread.Sleep(100);
        }
      }
  }