you are sorting on:
DATE_FORMAT(registered, '%m-%d') AS registered
you performing the formatting on the registered column and naming the result with the same name. This means that the resulting format is overriding the the previous timestamp format.
when it gets to the order by clause the data is no longer a timestamp, it is the formatted date.
you could change this to:
DATE_FORMAT(registered, '%m-%d') AS registeredMD
so that the original column format is not changed and the formatted date is returned as registeredMD.
you would then need to change your code to output the date:
<?php echo($row_WADAattendee['registered']); ?>
to:<?php echo($row_WADAattendee['registeredMD']); ?>
so that it is using the formatted date for display.