Touch.position, Touch.deltaPosition, Screen.width, Screen.Height

From a very famous Unity plugin source code, there is a line a don't quite understand:

 var touchZero = Input.GetTouch(0);
 var touchZeroPrevPos = touchZero.position - new Vector2(touchZero.deltaPosition.x / Screen.width, touchZero.deltaPosition.y / Screen.height);

what is purpose of touchZero.deltaPosition.x / Screen.width? Could Touch.position and Touch.deltaPosition have the same unit (pixel) ? Why not just touchZero.position - touchZero.deltaPosition?