confusing-this.js

function fun1() {
    setTimeout(function () { //normal
        console.log("normal (setTimeOut)", this);
    }, 0);
}
​
function fun2() {
    setTimeout(() => { //arrow
        console.log("arrow (setTimeOut)", this);
    }, 0);
}
​
var ob = {
    test1: () => console.log("arrow", this), // this => `window` --> it's okay...
    test2: function () {
        console.log("normal", this); // this => `ob` --> okay too...
    },
    test3: fun1, // this =>  `window` ---> Why???? 
    test4: fun2, // this => `ob` ---> Why?? O_o;
​
}
​
ob.test1();
ob.test2();
ob.test3();
ob.test4();