关于AS 3.0中父级的问题

一下是一个能实现小球转移的代码。
package {
import flash.display.Sprite;
import flash.events.MouseEvent;

public class parenting extends Sprite{
    private var _parent1:Sprite;
    private var _parent2:Sprite;
    private var _ball:Sprite;

    public function parenting() {
        // constructor code
        _parent1 = new Sprite ();
        addChild(_parent1);
        _parent1.graphics.lineStyle(1,0);
        _parent1.graphics.drawRect(-50,-50,100,100);
        _parent1.x = 60;
        _parent1.y = 60;


        _parent2 = new Sprite ();
        addChild(_parent2);
        _parent2.graphics.lineStyle(1,0);
        _parent2.graphics.drawRect(-50,-50,100,100);
        _parent2.x = 170;
        _parent2.y = 60;


        _ball = new Sprite();
        _parent1.addChild(_ball);
        _ball.graphics.beginFill(0xff0000);
        _ball.graphics.drawCircle(0,0,40); 
        _ball.graphics.endFill();

        _ball.addEventListener(MouseEvent.CLICK,onBallClick);
    }
    public function onBallClick(e:MouseEvent):void
    {
        _parent2.addChild(_ball);
    }

}

}

注意_parent1和_parent2，虽然在drawRect中已定义矩形坐标，但是随后代码又定义了坐标( _parent1.x = 60;
_parent1.y = 60;)。

问题：
为何我在运行时坐标却是引用drawRect的坐标？
此外如果去掉代码中的
_parent1.x = 60;
_parent1.y = 60;
和
_parent2.x = 170;
_parent2.y = 60;
则圆球转移的功能无法实现，请问这是为什么？