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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