TweenMax 怎么不是递减 元素的 x,y 属性数值?

Question

下面的代码,使用TweenMax的onUpdate方法

      this.detaibaseInfo2 = new Sprite();
            with (detaibaseInfo2) {
                x = 30;
                y = 700;
                with (graphics) {
                    beginFill(0x826b45, 1);
                    drawRect(0, 0, 240, 20);
                    endFill();
                }
            }
      addChild(detaibaseInfo2);

    TweenMax.to(detaibaseInfo2, 0.2, {y:260, onUpdate:detaibaseInfo2Update,ease:Strong.easeOut});

但是测试了一下:

    private function detaibaseInfo2Update():void {
      trace("detaibaseInfo2.y="+detaibaseInfo2.y);
      if (detaibaseInfo2.y < 270) {
          other code ...
      }
    }

输出的结果是:

[trace] detaibaseInfo2.y=399.7
[trace] detaibaseInfo2.y=399.7
[trace] detaibaseInfo2.y=399.7
[trace] detaibaseInfo2.y=399.7
[trace] detaibaseInfo2.y=399.7
[trace] detaibaseInfo2.y=291.45
[trace] detaibaseInfo2.y=263.25
[trace] detaibaseInfo2.y=260.05
[trace] detaibaseInfo2.y=260

有个疑惑: 为什么 detaibaseInfo2 的 y 属性数值不是从 700 到 260 的递减呢?