The duration of a task is working time based upon the calendar in use. So, if you have resources assigned to tasks - the calendar in use is their calendar. On summary tasks the project calendar (set in Project Information) is used to calculate duration.
In the attached picture, the only task shows a longer duration than the single summary task. The resource assigned to the task has a calendar with Saturday marked as working days. Based upon the resource's calendar, there are 21 working days between July 28 and August 20 - counting Saturdays. However the project calendar only has working days Monday through Friday - so there are only 18 working days using that measure.
When dealing with resources with different calendar definitions, it may be more meaningful to stop talking about "duration" and start talking start and finish dates plus work.
I hope this helps.
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