Joystick, touch

Hi, I'm using a standard joystick script but seem to be having some problems with it, while using it in iOS on the iPhone4 In landscape I can touch anywhere above centre screen and move around just as if I was using the joystick but more jerky .. anyone know why please ?

 #pragma strict
 
 var radius : float; //Radius of movement
 var deadZoneRadius : float; //Radius of dead zone within which the input value returned is 0
 
 private var gui : GUITexture; //Reference to the GUITexture component of this gameObject
 private var startingPosition : Vector2; //Starting position of this gui
 private var startingScreenCenter : Vector2; //Starting center screen position of this gui
 private var touchIndex : int = -1; //Index of the iPhone touch that hits this gui
 
 private var otherJoysticks : Joystick[];
 
 function Awake() {
 
  var childGUITextures;
  gui = GetComponent("GUITexture"); //Gather the reference for the GUITexture component
  if(gui.pixelInset.x > 240) { //If joystick is on the right-half of screen, move it to the right-edge if necessary
  var difference : int = 0;
  difference = Screen.width - gui.pixelInset.x - 120;
 
  childGUITextures = GetComponentsInChildren(typeof(GUITexture));
  for (var childGUI : GUITexture in childGUITextures) {
  childGUI.pixelInset.x += difference;
  }
  }
  
  //Calculate starting position and starting screen center
  startingPosition = Vector2(gui.pixelInset.x, gui.pixelInset.y);
  startingScreenCenter = Vector2(gui.pixelInset.x + gui.pixelInset.width * 0.5, gui.pixelInset.y + gui.pixelInset.height * 0.5);
  
  otherJoysticks = FindObjectsOfType(Joystick); //Gather references to all joysticks in scene
 }
 
 function Update () {
  var touch : Touch;
  if(touchIndex == -1) { //If a touch has not hit this gui since last time a touch was ended/canceled
  for(var i=0; i<Input.touchCount; i++) { //Go through all current touches
  touch = Input.GetTouch(i); //Get touch
  if(gui.HitTest(touch.position)) { //If this touch hits this gui
  touchIndex = i; //Set touch index to this touch
  for(var j=0; j<otherJoysticks.Length; j++) {
  if(otherJoysticks[j] != this)
  otherJoysticks[j].TouchTaken(touchIndex);
  }
  break; //We've found our touch so break out of the loop
  }
  }
  }
  if(touchIndex != -1 && touchIndex < Input.touchCount) { //If a touch has hit this gui and its index does not point to an invalid element in the current touch array
  touch = Input.GetTouch(touchIndex); //Get touch
  if(touch.phase == TouchPhase.Began || touch.phase == TouchPhase.Moved) { //If touch phase is Began or Moved
  var difference : Vector2 = touch.position - startingScreenCenter; //Calculate the difference between current touch position and the starting center screen position of this gui
  if(difference.magnitude > radius) { //If the length of the difference vector is greater than the radius of movement
  //Limit the distance of the difference vector so that input stays within the [-1..1] range
  var radians = Mathf.Atan2(difference.y, difference.x);
  difference.x = Mathf.Cos(radians) * radius;
  difference.y = Mathf.Sin(radians) * radius;
  }
  //Set the pixel inset of the GUITexture to show the movement of the joystick
  gui.pixelInset.x = difference.x + startingPosition.x;
  gui.pixelInset.y = difference.y + startingPosition.y;
  } else if(touch.phase == TouchPhase.Canceled || touch.phase == TouchPhase.Ended) { //If touch phase is Ended or Canceled
  touchIndex = -1; //Reset touch index to an invalid number
  //Reset the pixel inset of the GUITexture to its starting position
  gui.pixelInset.x = startingPosition.x;
  gui.pixelInset.y = startingPosition.y;
  }
  } else {
  touchIndex = -1; //Reset touch index to an invalid number
  //Reset the pixel inset of the GUITexture to its starting position
  gui.pixelInset.x = startingPosition.x;
  gui.pixelInset.y = startingPosition.y;
  }
 }
 
 //Returns the current input axis vector
 function GetAxis() {
  var axis = Vector3(gui.pixelInset.x, gui.pixelInset.y, 0) - startingPosition;
  return (axis.magnitude < deadZoneRadius) ? Vector3.zero : axis/radius;
 }
 
 //Returns whether a given touch position is contained within this gui
 function ContainsPosition(touchPosition : Vector2) {
  return gui.HitTest(touchPosition);
 }
 
 //Called by other Joysticks when a touch has been taken
 function TouchTaken(takenTouchIndex : int) {
  if(touchIndex == takenTouchIndex) { //If the touch that another Joystick took has the same index as this Joystick's touch
  touchIndex = -1; //Let the touch go
  //Reset the pixel inset of the GUITexture to its starting position
  gui.pixelInset.x = startingPosition.x;
  gui.pixelInset.y = startingPosition.y;
  }
 }