I am working on MS Project 2013. The summary task No. 6 is not showing the duration correctly. Refer to the picture below.
Summary task 6, Building Town planning, contains three unlinked subtasks of durations 200d, 300d, and 476d - all of which start concurrently. The duration of the summary task, at 476d, is exactly as expected - i.e. the difference between its start and finish dates according to the applicable calendar. Summary start and finish dates are rolled up from the subtasks. Since all the subtasks start at the same time, the summary's finish date and duration are both determined by the longest subtask - 476d.