No sort with except

Dear all,

When I run the following SQL with PostgreSQL 9.1:

--
SELECT DISTINCT ON (pernr) pernr, vorna, nachn, eindt, ausdt, updat,
status
FROM person

WHERE eindt <= TO_CHAR(CURRENT_DATE,'YYYYMMDD')
AND ausdt >= TO_CHAR(CURRENT_DATE,'YYYYMMDD')

ORDER BY pernr, eindt DESC;
--

it works. I get the most recent persons, even if one came back within this
time range.

But if i do this:

---
SELECT DISTINCT ON (pernr) pernr, vorna, nachn, eindt, ausdt, updat,
status
FROM person

WHERE eindt <= TO_CHAR(CURRENT_DATE,'YYYYMMDD')
AND ausdt >= TO_CHAR(CURRENT_DATE,'YYYYMMDD')
EXCEPT

SELECT DISTINCT ON (pernr) pernr, vorna, nachn, eindt, ausdt, updat,
status
FROM person
RIGHT JOIN table ON sobid = CAST (pernr AS VARCHAR) AND objid = 10
WHERE eindt <= TO_CHAR(CURRENT_DATE,'YYYYMMDD')
AND ausdt >= TO_CHAR(CURRENT_DATE,'YYYYMMDD')
ORDER BY pernr, eindt DESC;
---

In this case the ORDER BY does not work: I will get the same person data,
either with DESC as with ASC, even when this should change.

Does anyone have an explanation for this?

Many Thanks