The total costs for your task changed unexpectedly as the schedule progressed.
The task's total costs may have changed due to a task or resource schedule change that occurred, even though you have not entered new costs or updated the existing costs.
Verify the effective dates of resource rate changes that you set in the cost rate table in the Cost tab of the Resource Information dialog box. A rate change that you set to occur at a specific date may have gone into effect.
Verify the start date and end date of a task that you moved. If you moved a task into a different time frame, a different rate you set for that time frame may have been applied.