Calling function from within functions.php returns unwanted value

Inside this loop:

foreach ( $attachments as $id => $attachment ) {
        $link = isset($attr['link']) && 'file' == $attr['link'] ? wp_get_attachment_link($id, $size, false, false) : wp_get_attachment_link($id, $size, true, false);

        $output .= "<{$itemtag} class="gallery-item">";
        $output .= "<span class="photos-comment-number-wrap"><span class="comment-number">". comments_number() ."</span></span>";
        $output .= "
            <{$icontag} class="gallery-icon">
                $link
            </{$icontag}>";
        if ( $captiontag && trim($attachment->post_excerpt) ) {
            $output .= "
                <{$captiontag} class="gallery-caption">
                " . wptexturize($attachment->post_excerpt) . "
                </{$captiontag}>";
        }
        $output .= "</{$itemtag}>";
        if ( $columns > 0 && ++$i % $columns == 0 )
            $output .= '<br style="clear: both" />';
}

You can get at the comment count directly from the object:

$attachment->comment_count

Refer to the wp_posts table in the Database Description Codex entry for more information.