[SQL] Returning wrong ans [closed]

This is my query for the sql question from the sql 50 the question number is 1661.

select a.machine_id, round(avg(b.timestamp - a.timestamp),3) as processing_time
from Activity a
inner join Activity b
on a.process_id = b.process_id
and a.machine_id = b.machine_id
and a.timestamp < b.timestamp
group by machine_id;


and this is not accepted. It worked for the 7 testcases but on the 8th one the output first output not matches but the remaining outputs are correct how its possible as well this next code works fine and passes all the test cases can any body help me in understanding this

SELECT a.machine_id,
ROUND(AVG(b.timestamp - a.timestamp), 3) AS processing_time
FROM Activity a,
Activity b
WHERE
a.machine_id = b.machine_id
AND
a.process_id = b.process_id
AND
a.activity_type = 'start'
AND
b.activity_type = 'end'
GROUP BY machine_id;

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