AS3 painting problem

hi all, Merry Christmas

I’ve used to read and try,thanks kirupaForum for giving me so much help in my learning flash
I hope my 1st post can receive answers too

add the following to the timeline you will get what is puzzling me

//===================

import flash.events.;
import flash.display.
;
import flash.geom.;
import flash.utils.
;

//
var paintItemId:uint = 21;
var paintItemSize:uint = 10;
var paintItemColor = “FF0000”;
var paintItemAlpha:Number = 1;

var isPainting:Boolean = true;
var isPlaying:Boolean = false;

var subcanvasCount:int = 0;

//
var myCanvas_mc:Sprite = new Sprite();
myCanvas_mc.x = 0;
myCanvas_mc.y = 0;
myCanvas_mc.graphics.clear();
myCanvas_mc.graphics.beginFill(0xFFFFFF,0);
myCanvas_mc.graphics.drawRect(0, 0, 550, 400);
myCanvas_mc.graphics.endFill();
addChild(myCanvas_mc);

myCanvas_mc.addEventListener(MouseEvent.MOUSE_DOWN, onMyCanvasMouseDown);
//myCanvas_mc.addEventListener(MouseEvent.MOUSE_MOVE, onMyCanvasMouseMove);
//myCanvas_mc.addEventListener(MouseEvent.MOUSE_UP, onMyCanvasMouseUp);

function onMyCanvasMouseDown(evt:MouseEvent):void {
if (isPainting) {
var _subcanvas:Sprite = new Sprite();
_subcanvas.name = “subcanvas” + subcanvasCount;
myCanvas_mc.addChild(_subcanvas);

    subcanvasCount++;

    _subcanvas.graphics.clear();
    _subcanvas.graphics.beginFill(0xFFFFFF,0);
    _subcanvas.graphics.drawRect(0, 0, 576, 356);
    _subcanvas.graphics.endFill();
    _subcanvas.graphics.lineStyle( paintItemSize, paintItemColor,paintItemAlpha );

    if (paintItemId == 21) {
        _subcanvas.addEventListener(MouseEvent.MOUSE_DOWN, Id21MouseDown);
        _subcanvas.addEventListener(MouseEvent.MOUSE_UP, Id21MouseUp);
        _subcanvas.addEventListener(MouseEvent.MOUSE_MOVE, Id21MouseMove);

        function Id21MouseDown(evt:MouseEvent) {
            isPainting = true;
            _subcanvas.graphics.moveTo( evt.localX, evt.localY );

        }
        function Id21MouseMove(evt:MouseEvent) {
            if (isPainting) {
                _subcanvas.graphics.lineTo( evt.localX, evt.localY );
            }
        }
        function Id21MouseUp(evt:MouseEvent) {
            isPainting = false;
            evt.updateAfterEvent();
        }
    }

}

}

//===================

why all they start at (0,0)?