ScreenToViewportPoint offset from Input.mousePosition ?

ScreenToViewportPoint seems like it has an offset from the coords returned from mousePosition. How do you anticipate this offset or somehow make the btn right under the mouse cursor?

If you run the snippet below, you'd see that the btn is not right under the mouse cursor but off at a distance.

var btn:GameObject; var cam:Camera;

function Start(){ btn=GameObject.Find("btn"); cam=GameObject.Find("Camera").camera; } function Update () { btn.transform.position=cam.ScreenToViewportPoint(Input.mousePosition); }

in screenshot below, btn is the red gui element. evidently, it is offset from the mousePosition