180 engine +50 degree air will result in a higher reading that 180 engine +30 degree air. Also a tail wind will keep you much warmer than a cross wind. Was the higher temperature above ambient on the colder or warmer day?
I've got a good idea of what you've done. Makes me wonder if maybe you had a head wind and that upper grille block let more air pass or the opposite is true and the block on there made engine bay temps rise because of the engine working harder.