Filter results in different values with and without concat

Question

Can someone explain why there is a diff in output?

var a1 = [1, 2, 3, 5];
var a2 = [1, 2, 3, 4, 5];
var a3 = [];

a3.concat(a2.filter(n => a1.indexOf(n) === -1));
console.log(a3); // a3 = []

a3 = a2.filter(n => a1.indexOf(n) === -1);
console.log(a3); // a3 = [4]