在闭包内的“this”关键字上

我知道“这个”在封口的外面和里面是不同的。
但为什么是小孩子这个。numChildren封口内部有什么不同?

var _this:Sprite = this;
trace("[outside]this: " + this);
trace("[outside]numChildren: " + numChildren);
trace("[outside]this.numChildren: " + this.numChildren); 

(function ():void {
    trace("[inside]this: " + this);
    trace("[inside]numChildren: " + numChildren);
    trace("[inside]this.numChildren: " + this.numChildren);
    trace(_this.removeChildAt === removeChildAt);
    trace(this.removeChildAt === removeChildAt);
})();

您可以从下面的链接中看到代码和输出
How Do You Explain 'this'?