javascript 函数返回值的问题，请高手解答

Question

function aFun(){

    function _(arg){

        this.arg = arg;

    }

    _.prototype.testFun = function(){

        console.log('this is prototype testFun')

    };

    _.testFun2 = function(){

        console.log('this is testFunc2');

    }

    return _;

};

var afun = new aFun('123');

console.log(afun instanceof aFun)

afun.testFun2(); //可以调用

afun.testFun(); //不是在原型链上么？，为啥不可以调用？？？，

var aFun2 = (function (){

    function _(arg){

        this.arg = arg;

    }

    _.prototype.testFun = function(){

        console.log('this is prototype testFun')

    };

    _.testFun2 = function(){

        console.log('this is testFunc2');

    }

    return _;

})();

var afun2 = new aFun2('ddd');

console.log(afun2 instanceof aFun2)

afun2.testFun2(); //不可以调用，为啥？

afun2.testFun(); //可以调用，为啥可以？（我要疯了）