It's undoubtedly more than that, depending on where you measure. The gradient between e.g. the roof lining and the AC vent could easily be 20C+ degrees on a hot day. Most boards on a vehicle will have their own temperature sensors to measure enclosure temps, and there will be zonal sensors at various points in the cabin as well. The climate control loop will be defined in terms of those zonal sensors.
Measuring temperature is not trivial. There's convective, conductive and radiative heat transfer and they all factor into the measurement. And that may not accurately reflect the "cabin temperature," particularly in a parked car.
