strange (numeric) casting behaviour

I'm experience some strange behaviour when casting numeric values.

Given the following SQL-Statement:

SELECT
a_int,
a_num,
CAST (a_num AS NUMERIC(9,1)) AS CastTo9_1,
CAST (a_num AS NUMERIC(9,2)) AS CastTo9_2,
CAST (a_num AS NUMERIC(9,3)) AS CastTo9_3
FROM f_numtest();

Generated output is:
 a_int |       a_num        | castto9_1 |     castto9_2      | castto9_3
-------+--------------------+-----------+--------------------+-----------
  1525 | 8.6317245901639344 |       8.6 | 8.6317245901639344 |     8.632

Expected output was:
 a_int |       a_num        | castto9_1 |     castto9_2      | castto9_3
-------+--------------------+-----------+--------------------+-----------
  1525 | 8.6317245901639344 |       8.6 |          -->  8.63 |     8.632



reproducable testcase:

CREATE TABLE numtest (
  n NUMERIC(9,2),
  i INTEGER
);
INSERT INTO numTest VALUES (13163.38000, 1525);

CREATE TABLE frslt_test (
  a_int INTEGER,
  a_num NUMERIC(9,2)
);

CREATE OR REPLACE FUNCTION f_numTest() RETURNS SETOF frslt_test AS '
SELECT  CAST (SUM(i) AS INTEGER),
SUM(n)/SUM(i)
FROM Numtest
' LANGUAGE 'sql';

SELECT
a_int,
a_num,
CAST (a_num AS NUMERIC(9,1)) AS CastTo9_1,
CAST (a_num AS NUMERIC(9,2)) AS CastTo9_2,
CAST (a_num AS NUMERIC(9,3)) AS CastTo9_3
FROM f_numtest();

select version();

 a_int |       a_num        | castto9_1 |     castto9_2      | castto9_3
-------+--------------------+-----------+--------------------+-----------
  1525 | 8.6317245901639344 |       8.6 | 8.6317245901639344 |     8.632

                                                  version
------------------------------------------------------------------------------------------------------------
 PostgreSQL 8.1.3 on i486-pc-linux-gnu, compiled by GCC gcc-4.0.gcc-opt (GCC) 4.0.3 (Ubuntu 4.0.3-1ubuntu4)