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)