The heat is lost on radiational cooling nights on object surfaces...the ground, car roof, house roof, etc. With no advection, 2m gets colder from the bottom-up from the ground surface. Black bodies will radiate more than white. It's why you get the frost forming on the car or ground first before the 2m temp reaches 32F.
We want the sensor to be representative of the free standing air at 2m...not the temperature of the object (whatever material it may be) that is rapidly losing heat to space.