Question 1: What are the factor that play a role in this configuration?
My guess is radiation from the building and the wind (the frost free part being more protected from the wind by the building) but then I wonder : Will there still be these two zones after a night without any wind? (Question 2)
The left house is heated, the middle one isn't but it's inhabited, the shed is closed but not insulated and without heating source inside. There isn't any other surrounding structures that could create shade.
The open side of the garden is facing south.
I observed the frost after sunrise, but before the sun rays were reaching the garden.