Some departments (operational) have productivity as their main target, so when the cost driver changes this will give the department more or fewer hours depending on the latest live forecast.
Some departments have hours as their main target (Admins) so even if activity changes, the expected hours will not change. Under settings in the cockpit, you can see if a department is working by productivity or hours.
Example: You had more rooms to clean, but the same amount of staff to clean them, the staff then needs to clean more rooms in the same amount of time increasing their productivity