Thank you very much both “alphaoide” and “jaredquinn”. Finally I realised it is a bug. Hope it would get fix soon.
Next, I have already success in preventing the counter show in preview pane by adding this code.
<?php if ( !isset($_SERVER[‘HTTP_REFERER’]) || $_SERVER[‘REQUEST_URI’] == ‘/’ || substr($_SERVER[‘HTTP_REFERER’], strlen(get_bloginfo(‘url’)), 36) != ‘/wp-admin/post.php?action=edit&post=’ ) { ?>
…CODE…
<?php } ?>
Thank you very much everybody for your wisdom and your share.