насчёт запоминания this

[aQuestion]

var F:Function = function () {
	this.f = function () {
		trace (this);
	};
	this.toString = function () {
		return "hello";
	};
};
var o:Object = new F (); o.f ();

пишет hello

function G ():Function {
	var F:Function = function () {
		this.f = function () {
			trace (this);
		};
		this.toString = function () {
			return "hello";
		};
	};
	var o:Object = new F ();
	return o.f;
}; G () ();

пишет [object global]

function G ():Function {
	var F:Function = function () {
		this.f = function () {
			trace (this);
		};
		this.toString = function () {
			return "hello";
		};
	};
	var o:Object = new F ();
	return o.f;
}; var z:Function = G (); z ();

пишет [object MainTimeline]

Почему?