do_action issue with parameters handling

Ok, so I wanted to hook up to an action post_updated.

/* Line 2654 wp-includes/post.php /*
do_action( 'post_updated', $post_ID, $post_after, $post_before);

As you can see, there are 3 parameters. But when you look at do_action definition in wp-inludes/plugin.php you can notice.

/* Line 386 */
$args = array();
	if ( is_array($arg) && 1 == count($arg) && isset($arg[0]) && is_object($arg[0]) ) // array(&$this)
		$args[] =& $arg[0];
	else
		$args[] = $arg;
	for ( $a = 2; $a < func_num_args(); $a++ )
		$args[] = func_get_arg($a);

Notice the for loop with $a set to 2. It means, that this will omit next 2 parameters.
Is this made on purpose?