Query problem

while working with and testing my new favorite query,
i ran into some behavior that i didn't expect.

the query is as follows:

SELECT DISTINCT t_product.product_id,
       t_product.product_number,
       t_serial_number.serial_number_id,
       t_serial_number.serial_number::int
FROM t_serial_number
LEFT JOIN t_link_contract_number_job_number
    ON (
t_serial_number.link_contract_number_job_number_id =

t_link_contract_number_job_number.link_contract_number_job_number_id
       )
LEFT JOIN t_job_number
   ON (
t_link_contract_number_job_number.job_number_id =
        t_job_number.job_number_id
      )
LEFT JOIN t_product
   ON ( t_product.product_id =
        t_job_number.product_id
      )
LEFT JOIN t_inspect
   ON ( t_serial_number.serial_number_id =
        t_inspect.serial_number_id
      )
LEFT JOIN t_inspect_area
   ON ( t_inspect.inspect_area_id =
        t_inspect_area.inspect_area_id
      )
WHERE t_serial_number.serial_number_id NOT IN
      (SELECT serial_number_id FROM t_inspect
       WHERE t_inspect_area.inspect_area_id = 1
       AND inspect_pass = true)
ORDER BY t_serial_number.serial_number::int ASC

my purpose is to find the serial number that doesn't
have at least one TRUE inspect_pass value by serial
number / inspect area.

starting with no data:  i enter in any number of
production inspect (t_inspect_area.inspect_area_id =
1) fails for serial number 1 (s/n 1).  when i query
inspect_id = 1, s/n 1 returned as expected.  i enter a
true value for t_inspect_area.inspect_area_id = 1 and
s/n 1 no longer returns when i query inspect_area_id =
1 - just as expected.

the problem begins when i move to qa inspection
(t_inspect_area.inspect_area_id = 2) and enter a true
value for s/n 1.

not only does a query for inspect_id = 2 (qa
inspect_area) return s/n 1, now a query based on
inspect_id returns s/n 1.

i'm stumped as to why.  any help would be appreciated.

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