Limiting the number of posts in WP_Query leads to unexpected result

The following is supposed to display 10 posts.

Sometimes it displays 11, sometimes 12, and sometimes the desired 10 posts. It behaves the same if I swap posts_per_page for showposts or numberposts .

Where do I need to search for the problem?

<ul>
    <? wp_reset_query(); ?> 
    <?  $args = array(
        'orderby'           => 'rand',
        'posts_per_page'    => '10',
    );
    $query_footerFavorite = new WP_Query($args);

    while ($query_footerFavorite->have_posts()) : $query_footerFavorite->the_post(); ?>

        <li>
            <a href="<? the_permalink(); ?>" title="<? the_title_attribute(); ?>">
                <? the_title(); ?>
            </a>
        </li>

    <? endwhile; ?>
    <? wp_reset_query(); ?>                 
</ul>

Edit: It seams there is a problem with the posts_per_page, see print_r($query_footerFavorite):

WP_Query Object ( [query_vars] => Array ( [orderby] => rand [posts_per_page] => 10 [error]