The reason the two may not show identical is assignments in the Gradebook allow you to specify a Due Time as well as date, while the report only allows you to specify by date. The report uses 12 a.m. as the time for both fields, so any assignments due after 12 a.m. on the End Date selected while running the report will not be included in the report's calculations.

In order to make sure your cumulative's match, select one day later for the end date than your Marking period ends.This will insure all assignments due the last day of the marking period are included in the report.

Example: If the marking period goes from 9/1/2015 to 11/1/2015, set the start and end dates for the report to 9/1/2015 and 11/2/2015.