Javascript module pattern - Object.create

A bit of confusion regarding what exactly is going on here.

given this simple code :

stuff = (function(){
    function extendFoo(bar)
    {
        $.extend(this.foo,bar);
    }
    this.foo = {};

    return { foo : foo, extendFoo: extendFoo};
})();

Following this simple operation :

zz = Object.create(stuff);
zz.extendFoo({bar:'bar'});
vv = Object.create(stuff); //foo is still extended(??)

So from what I realized, operations conducted on the returned object form Object.create still affect the prototype of that Object, So when you create a new object, his prototype is changed, thus resulting in a 'modified' version.

This seems wrong on a lot of levels, Can anyone explain what's going on here?

this behavior is not reproducible using the following pattern :

stuff = (function(){

    function setFoo(bar)
    {
        this.foo = bar;
    }
    var foo;

    return { foo : foo, setFoo: setFoo};

})();

So I suspect $.extend is to blame here.

Any input would be great!