Scenario:
In the Project status report Sum of hours does not equal the hours listed. There is a discrepancy in the Sum of the Total Hours. Sum of the total hours is showing as 3913.00 however when calculated it should be 4011.00.

Resolution:
The report is calculating the sum correctly. If any user is assigned to multiple department then in case of Department filter is active it only shows the total hours entered for that department only.
The report is currently correct. The relationship here is Department -> User -> Timesheet, and the main point is that this report uses the user's current departments not the departments the user was assigned to when the Timesheet was created. To understand, take a look at the report in question but enable the User Name column, and sort the report by User Name. It's that the hours in this report are by user, not by department. The users current department is joined in to display department.

When a user is assigned to more than one department you see that all the departments are listed in the report, although the hours you see have only been entered once. This is the reason for what appears as a discrepancy, because the summary is intelligent enough to know that while the departments are listed separately, the hours listed were only entered once so are only included in the summary total once.

For example:
User Robin is added to 3 departments, and has entered 80 hours for the date range provided. The report will show us:
UserName ClientName DepartmentName TotalHrs
Test Client #1 Department #1 80
Test Client #1 Department #2 80
Test Client #1 Department #3 80
Summary 80

The hours entered by the user are duplicated for each department that user is assigned to, so the summary is smart enough to know that those hours are duplicates and not include them in the summary. This is the reason for the discrepancy, but the summary is an accurate total. Try running the report with username enabled and you will be able to spot some duplicates.