There is a small difference between the MouseOut event and the RollOut event.
MouseOut event is not containers aware. RollOut event is container aware.
MouseOut event is fired by the Flash Player and is not aware of containers and children. This means that, if you are listening to MouseOut event on a container and the you move your mouse over a child of that container the MouseOut event is fired.
Where as the RollOut event understands the containers and will only fire when the mouse is rolled out of the container and not on to any of its children. You can find more details at the URL below
http://livedocs.adobe.com/flex/3/langref/flash/display/InteractiveObject.html#event:rollOut