Confused by behavior of `map` on arrays created using `new`

I am confused by the results of mapping over an array created with new:

function returnsFourteen() {
    return 14;
}

var a = new Array(4);
> [undefined x 4] in Chrome, [, , , ,] in Firefox
a.map(returnsFourteen);
> [undefined x 4] in Chrome, [, , , ,] in Firefox

var b = [undefined, undefined, undefined, undefined];
> [undefined, undefined, undefined, undefined]
b.map(returnsFourteen);
> [14, 14, 14, 14]

I expected a.map(returnsFourteen) to return [14, 14, 14, 14] (the same as b.map(returnsFourteen), because according to the MDN page on arrays:

If the only argument passed to the Array constructor is an integer between 0 and 2**32-1 (inclusive), a new JavaScript array is created with that number of elements.

I interpret that to mean that a should have 4 elements.

What am I missing here?