Displaying documents attached to parent page

I’m pretty clueless with PHP, but I put together a wp_query for a client’s site that displays all the PDF files attached to the current page–and its parent. It works perfectly, except for a weird glitch: A handful of documents also appear on pages that have no attachments and no parent. Can anyone more proficient in the code than I am discern what might be causing the trouble?

// Get the slug of the parent page.
if($post->post_parent) { $post_data = get_post($post->post_parent);
$post_parent_slug =  $post_data->post_name; }
?>

<?php // Documents attached to the parent page: 

	// The Query
    $the_parents_query = new WP_Query( array( 'post_type' => 'page', 'name' => $post_parent_slug ) );

	// The Loop
    while ( $the_parents_query->have_posts() ) : $the_parents_query->the_post(); 

	$parentargs = array(
   'post_type' => 'attachment',
   'post_mime_type' => 'application/pdf,application/msword', // Only show PDFs or DOCs.
   'numberposts' => -1,
   'post_status' => null,
   'post_parent' => $post->ID
  );

  $attachments = get_posts( $parentargs );
     if ( $attachments ) {
        foreach ( $attachments as $attachment ) {
           echo '<li>Download: <a title="Download Document" href="';
		   echo wp_get_attachment_url( $attachment->ID );
		   echo '">';
		   echo apply_filters( 'the_title', $attachment->post_title );
		   echo '</a></li>';
          }
     }

 endwhile; ?>