$charset = $_POST['charset'];
$title = mb_convert_encoding($title, get_settings('blog_charset'), $charset);
So WP use the 'charset' field in post data as the from encoding. But in trackback() function (wp-includes/functions.php), it does not send a charset field in post data.
(Yes, it send a charset in Content-Type: ... charset: , but that's not enough. )