Variables tag_slug__and query_post does not work

Earlier,

I had my script as follows:

$what_tag = get_post_meta($post->ID, 'what_tag', $single);
$post_args = array('tag_slug__and' => array($what_tag)));
$lastposts = query_posts($args);

The script was suppose to display the tag slugs that were inputed from a custom field.

This didn’t work.

Instead, I had to do it this way:

$what_tag = explode(',',get_post_meta($post->ID,'what_tag',TRUE));
$args = array('tag_slug__and' => $what_tag)));
$lastposts = query_posts($args);

Is this something to do with WordPress or a PHP thing?