Result of `document.getElementsByClassName` doesn't have array methods like `map` defined, even though it is an array

Question

I have the following bit of code to select some divs and add a click handler on them

var tiles = document.getElementsByClassName("tile");

tiles.map(function(tile, i){
    tile.addEventListener("click", function(e){
        console.log("click!");
    });
});

This throws an error because map is not defined, even though tiles is an array. If I make an array like this, then map works fine:

var a = [1, 2, 3, 4];
a.map(/*whatever*/);

A workaround is to attach map to tiles like this:

tiles.map = Array.prototype.map;

This works fine. My question is why doesn't tiles have map defined on it? Is it not really an array?