rtrim giving weird result

I'm running Postgres 7.0.3 on a RedHat Linux 6.1. For some reason, rtrim
is giving me an incorrect result:

db01=# SELECT tablename FROM pg_tables WHERE tablename LIKE '%_opto' AND

tablename NOT LIKE 'pg%' ORDER BY tablename ASC ;
tablename
-----------------
center_out_opto
circles_opto
ellipse_opto
ex_ellipse_opto
figure8_opto
ro_ellipse_opto
(6 rows)

Now I want to return the same thing only with the trailing '_opto'
removed:

db01=# SELECT rtrim(tablename, '_opto') FROM pg_tables WHERE tablename
LIKE '%_opto' AND tablename NOT LIKE 'pg%' ORDER BY tablename ASC ;
rtrim
------------
center_ou <=======================
NOTE: the trailing 't' is missing
circles
ellipse
ex_ellipse
figure8
ro_ellipse
(6 rows)

However, as you can see, the 'center_out' table is missing the last 't'.
If I exclude the '_':

db01=# SELECT rtrim(tablename, 'opto') FROM pg_tables WHERE tablename
LIKE '%_opto' AND tablename NOT LIKE 'pg%' ORDER BY tablename ASC ;
rtrim
-------------
center_out_
<======================= 't' shows up again
circles_
ellipse_
ex_ellipse_
figure8_
ro_ellipse_
(6 rows)

The 't' is back.

Is there something that I'm doing wrong with my query here?

Thanks.
-Tony