ทำไมผมลบ session แล้วเวลาเอามาแสดงกลับเป็นแบบนี้หล่ะครับ

<?
ob_start();
session_start();
$_SESSION["a"] = array(a,b,c,d);
?>

<?
Array
(
    [a] => Array
        (
            [0] => a
            [1] => b
            [2] => c
            [3] => d
        )

)
?>

<?
ob_start();
session_start();

for ($i = 0; $i < count($_SESSION["a"]); $i++){

	echo "<br>".$_SESSION["a"][$i];
}

?>

<?
ob_start();
session_start();

unset($_SESSION["a"][1]);
?>

<?
Array
(
    [a] => Array
        (
            [0] => a
            [2] => c
            [3] => d
        )
)
?>