I’m pretty sure it’s because with a mouse as the input type the X/Y coordinates are relative to the screen.
The “MouseMovement” UserInputType will fire for whichever input device is considered the “mouse”. In the case of an Xbox controller it’d be one of the thumbsticks. (It depends on your settings I’m fairly sure.) Therefore, the input X/Y would show the position of the thumbstick, which is between -1 and 1 on each axis respectively. (Full circle.)