get_post_meta ads an extra “array”?

Hello
When I save an array with get_post_meta I’ll get an extra array as this:

Array
(
    [0] => Array
        (
            [0] => http://www.yahoo.com
            [1] => http://www.internet.com
            [2] => http://www.google.com
            [3] => http://www.cnn.com/
            [4] => http://www.php.net/
        )

)

So when I gonna write it out I have to write like this:

$a = get_post_meta(1,'my_key');

for($i=0;$i<count($a[0]);$i++) {
    echo $a[0][$i].'<br />';
}

Why this?