Hmmmm…interesting it’s doing that. WordPress must have changed something because the code that I used for that is from the Twenty Twelve theme. The code is found in the /includes/comment-form.php file and looks like this:
printf( '%1$s %2$s',
// If current post author is also comment author, make it known visually.
( $comment->user_id === $post->post_author ) ? ' ' . __( '(Post author)', 'celestial' ) . '' : ''
The part that does the printed label (Post author) is in that last line.
If you are using a child theme, you can copy the comment-form.php and upload it to your child theme into the “includes” folder there (create one if not there), then either remove that batch of code or re-label Post author as Comment Author.
I made a note to take this out because it apparently is not working anymore by WordPress…perhaps changes happened in version 3.8
NOTE: Actually, you can take that out of the parent theme because I will do an update for Celestial Reloaded.