Its because he is formatting it as a username. Change the following:
To:
You can use $post['lastip'] to display the last know IP as well.
You should ideally use the template system instead as already mentioned.
<td class=\"trow1\" align=\"center\">".build_profile_link($user['regip'], $user['uid'])."</td>
To:
<td class=\"trow1\" align=\"center\">".my_inet_ntop($db->unescape_binary($user['regip']))."</td>
You can use $post['lastip'] to display the last know IP as well.
<td class=\"trow1\" align=\"center\"><b>Registration IP:</b> ".my_inet_ntop($db->unescape_binary($user['regip']))."<br /><b>Last Known IP:</b> ".my_inet_ntop($db->unescape_binary($user['lastip']))."</td>
You should ideally use the template system instead as already mentioned.