Probleme avec mouseX

A partir de l'exemple suivant, j'aimerai comprendre pourquoi mouseY varie de 0 a 99 (ce qui me semble normal) alors que mouseX varie de 1 a 100 (ce qui me semble anormal).

package {
import flash.display.*;
import flash.events.*;
import flash.text.*;
import flash.geom.*;

[SWF(width='500',height='200')]

public class essaicoord extends Sprite {

public var t:TextField;
public var sp:Sprite;

public function essaicoord () {
super ();
x = 0;
y = 0;
t = new TextField ();
t.x = 0;
t.y = 0;
t.height = 20;
t.width = 500;
t.text = "Cliquer puis bouger sur le carre blanc";
addChild (t);
sp = new Sprite ();
with (sp.graphics) {
beginFill (0xFFFFFF);
drawRect (0,0,100,100);
endFill ();
}
sp.x = 30;
sp.y = 50;
sp.addEventListener (MouseEvent.MOUSE_DOWN,onDown);
sp.addEventListener (MouseEvent.MOUSE_UP,onUp);
addChild (sp);
}

private function onDown (e:MouseEvent):void {
sp.addEventListener (MouseEvent.MOUSE_MOVE,onMove);
stage.addEventListener (MouseEvent.MOUSE_UP,onUp);
onMove (e);
}

private function onUp (e:MouseEvent):void {
sp.removeEventListener (MouseEvent.MOUSE_MOVE,onMove);
stage.removeEventListener (MouseEvent.MOUSE_UP,onUp);
}

private function onMove (e:MouseEvent):void {
t.text = new Point (sp.mouseX,sp.mouseY).toString();
}
}
}

Merci