Stange meta_query results

Hallo!
I thought that one could use meta_query to find fields which have a certain meta_key and meta_value. Hovever, when I put this code somewhere on my webpage:

$q = new WP_Query( array(
    'meta_query' => array(
        array(
            'key' => 'Frog',
            'value' => 'Duck',
        )
    ),
));
while ($q->have_posts() ) {
    $p = $q->next_post();
    echo $p->ID . '<br>';
}

this prints “6
139″ although I have no post with meta_key “Frog” neither a post with meta_value “Duck”. Altogether, I have 174 distinct post_id’s in my table wp_postmeta, and I have no idea what is special about post no. 6 and no. 139.

Ideas?
Philipp