How are the summary task values calculated?
Not all summary task values show the combined total of your subtask values. Although some summary task values (such as cost and work) represent the total subtask values, others (such as duration and baseline) do not. For example, application calculates the duration of a summary task as the total work time between the earliest start date and the latest finish date of its subtasks.
The summary task information summarizes the period between the earliest start date and the latest finish date of all the included subtasks. It does not show the sum of all the subtask durations. Because application calculates the summary task values, they cannot be edited. If you need to change a summary task’s values, such as duration, update the individual subtasks. Summary task values are then recalculated.