[9-slice scaling] not scaling at correct size?

Hi people,

I found something strange with the 9-slice scaling option.
My MC with that option is simply smaller than the one without it !
You can see an example here (fla in attachement).
with the simple code on the green button :

on (release) {
	_root.i += 10;
	this.scroller._height = _root.i; // the left one with 9ss
	this.bar._height = _root.i; // the right one without 9ss

Does someone have any explanation ?