Why do let and var produce different results in this closure?

Question

The attached code returns 3, 3 & 3. If I change var to let, it returns 1, 2 & 3.

Why is this?

I expected both to return 1, 2 & 3. I understand that Var is function scoped, and let is block scoped - but how does that play out here?

function buildFunctions () {
    var arr = [];

    for (var i = 0; i < 3; i++) {

        arr.push(
            function () {
                console.log(i);
            }
        );
    }

    return arr;
}

let fs = buildFunctions();

fs[0]();
fs[1]();
fs[2]();