The Actual cost in a report is a product of Actual Hours (Selected dates) and Hourly cost. Actual hours is taken from the time entered in the timesheet by the user. The Hourly cost is set up in the users profile.
You must use actual costs within a timesheet detail report in order for it to use the rate in place at a particular time. if you use a project report, it will only be able to access the actual rates in place at current time you run report.