Because it's a "Level of Effort" Summary Task Calculation.

When assigning a summary task to a resource, Project Insight also assigns additional work to that resource. The work assigned to the resource is NOT the total of all work within the summary. In fact, it is ADDITIONAL work to the work within its corresponding tasks. In other words, you not only add up all of the work from the tasks to the summary, but you also add more hours from the summary task to itself.

Task 1 and Task 2 have a total work value of 48 hours. The summary, which calculates to a total duration of 5 days gets an additional value of 40 hours work assigned to it (one resource is assigned to the summary at 100% allocation in a 40 day work week). As a result, the summary calculates to 88 total hours.

Although this is often perceived as an incorrect calculation by those who have mistakenly applied it, it is correct per PMI calculations. Resources assigned to summary tasks are assigned as "level of effort" resources. Those who do wish to assign level of effort calculations to summary tasks will find that it does work quite well for resource allocation to level of effort tasks.