Simulate count result are distinct between 8.3 and 8.4

hi people!

I have this test query to simulate rownums in 8.3:

SELECT
(select count(i)+1 from prueba
where i < xi.i
) as rownum, i, p
FROM prueba xi limit 5;

Devuelve , que esta bien:
1;1;"299361"
2;2;"421127"
3;3;"166284"
4;4;"458945"
5;5;"81619"


But in 8.4 throws this:

postgres=# SELECT
postgres-# (select count(i)+1 from prueba
postgres(# where i < xi.i
postgres(# ) as rownum, i, p
postgres-# FROM prueba xi limit 5;
 rownum |   i    |   p
--------+--------+--------
 168770 | 168763 | 908731
 168771 | 168764 | 640826
 168772 | 168765 | 571112
 168773 | 168766 | 992462
 168774 | 168767 | 992471
(5 filas)


synchronize_seqscan are on in both engines.


I only want to know why is this difference...

--
              Emanuel Calvo Franco
             Database consultant at:
                    www.siu.edu.ar
        www.emanuelcalvofranco.com.ar