I have a really strange error with getting the screen size

private var bildBreite = Screen.width;
private var bildHoehe = Screen.height;
function OnGUI(){
Debug.Log(bildBreite);
Debug.Log(bildHoehe);
}

This script give me back width = 1252 (Correct) and height = 30?? (Sometimes 20??)

Instead when I directly use Debug.Log(Screen.height); it is correct?

Whats this for a really strange problem?

Thanks