Why this query does not work?

This works:

service0=# SELECT DISTINCT TO_CHAR(session_start, 'Month') as start,
service0-# TO_CHAR(session_start, 'Month') as
end
service0-# FROM user_sessions
service0-# WHERE TO_CHAR(session_start, 'YYYY') = '2003';
start | end
-----------+-----------
April | April
(1 row)

Why this does not gives correct result?

service0=#
service0=# SELECT DISTINCT machine_id
service0-# FROM user_sessions
service0-# WHERE TO_CHAR(session_start, 'Month') = 'April';

machine_id
------------
(0 rows)

When this does have no problem?

service0=# SELECT DISTINCT machine_id
service0-# FROM user_sessions
service0-# WHERE TO_CHAR(session_start, 'Month') = (SELECT
DISTINCT TO_CHAR(session_start, 'Month')
service0(# FROM
user_sessions);

machine_id
------------
123
(1 row)

Thank you for your time.

DanielD

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