As you would expect, coordinate positions of the mouse are provided in mutually exclusive ways in Netscape and MSIE. This is the MSIE equivalent to the layer coordinate scheme in Netscape. Of course, MSIE does not have layers but they can be simulated by using arbitrary objects.
The value in this property is calculated relative the coordinate system of the container of the source element that generated the event. This may not be the same as the coordinate scheme for the receiving object.