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