'this' scope on arrow fn

suprsie.js

var a = 2;

var Obj = {
  a: 4,
  test: () => {
    console.log(a);
    console.log(this.a);
	},
  test2: function({a = 6}) {
    console.log(a);
    console.log(this.a);
  } 
}

Obj.test2({a: 2}) // this.a nice and correct
Obj.test(); // wtf?