If the ambient air temperature is warmer when you start your HHR than when you shut it off the last time, you must drive the vehicle for a certain amount of time before the BCM updates the displayed temperature.
If the ambient air temperature is cooler when you start your HHR than when you shut it off the last time, the BCM displays the actual ambient air temperature immediately. This way, the engine controls are more efficient. There is less chance of a conflict between the outside air temperature and the coolant temperature readings, both of which are inputs involving emission controls. It is important that the ambient air temperature sensor readings be accurate - it is not an "FYI" by any means.