Though you lose cold air when you open the door of the fridge
, the items in the fridge
are still at the pre-opening of the door temperature. They respond slowly to opening the door where the air changes over quickly. Most fridge thermometers are reading air temp.
The items in the fridge have higher thermal capacity than the air (items 'hold more cold' like ice holds more cold than air). As a result reading the air temperature inside the fridge is not indicative of the temperature of the items in the fridge.
The 'more full' the fridge, the less effect opening the door has on the temp of the items.