2D rotate object towards mouse?

So I got this code snippet off another answer but for some reason its always off by exactly 90 degrees. On line 9 I added a hot fix(just takes the angle and minus 90) but it still bothers me anyone know why?

 public var angle : float;
 function Update () {
     var mousePos = Input.mousePosition;
     mousePos.z = 10.0f; //The distance from the camera to the player object
     var lookPos : Vector3 = Camera.main.ScreenToWorldPoint(mousePos);
     lookPos = lookPos - transform.position;
     angle = Mathf.Atan2(lookPos.y, lookPos.x) * Mathf.Rad2Deg;
     angle -= 90;
     transform.rotation = Quaternion.AngleAxis(angle, Vector3.forward);
 }