Why is PostgreSQL calling my STABLE/IMMUTABLE function multiple times?

I'm trying to optimise a complex query in PostgreSQL 9.1.2, which calls some functions. These functions are marked STABLE or IMMUTABLE and are called several times with the same arguments in the query. I assumed PostgreSQL would be smart enough to only call them once for each set of inputs - after all, that's the point of STABLE and IMMUTABLE, isn't it? But it appears that the functions are being called multiple times. I wrote a simple function to test this, which confirms it:

CREATE OR REPLACE FUNCTION test_multi_calls1(one integer)
RETURNS integer
AS $BODY$
BEGIN
    RAISE NOTICE 'Called with %', one;
    RETURN one;
END;
$BODY$ LANGUAGE plpgsql IMMUTABLE;


WITH data AS
(
    SELECT 10 AS num
    UNION ALL SELECT 10
    UNION ALL SELECT 20
)
SELECT test_multi_calls1(num)
FROM data;

Output:

NOTICE:  Called with 10
NOTICE:  Called with 10
NOTICE:  Called with 20

Why is this happening and how can I get it to only execute the function once?