For argument's sake: air flow when parked in a garage is reduced. I've observed it directly, and could calculate it if needed. I measured my own 3car garage air climbing over 120F after for 15-20 mins at the end of my commute. The air being pulled across the radiator, however, is poorly controlled by these chassis. When revving the engine, it would blow any small debris on my floor FORWARD. Now, I never had my thermometer right in front of the car (because I already knew idling in an enclosed space was dumb) but stepping across the front of the car it was apparent it was well beyond the ambient which was measuring 120 - Was it 130? who knows, but it was hotter for sure.
Also, anecdotally, the closer my grille whatever was directly ahead of my parked car (I had some stuff stored there which varied in space consumed), the faster the temp rose. This correlates with the air being blown forward too.
So it's not just a measure of ambient temp, it's available airflow (which is restricted) and the recycling of a local pocket of hot air that gets even hotter than ambient.
In open air, the recirculation is reduced substantially and the total amount of air flowing across the radiator is also increased.
