screen availWidth and Height

[wxdqz]

Could someone tell me why:

<script language="javascript">
<!--
document.write(screen.availHeight+"<br>");
document.write(screen.availWidth);
-->
</script>

Will write the correct avail screen height and width but:
window.open('','','width=screen.availWidth,height=screen.availHeight')
will not open a window of those dimentions.
???