Why is it localised?

Why is <th scope="row"><label for="link_rel"><?php /* translators: xfn: http://gmpg.org/xfn/ */ _e('rel:') ?></label></th> in wp-admin/includes/meta-boxes.php:1102 localised?

That text <?php /* translators: xfn: http://gmpg.org/xfn/ */ _e('rel:') ?> clearly shouldn’t be localised, it refers to a attribute which is the same in all languages (rel="")

Am I missing something?

Yes; the string being translated here is actual text rel:, and not HTML attribute rel="". I think the colon would be on the wrong side of the word in a RTL language and, thus, it requires translation.