Usually when tops are frosted off like that it means it was a clear still night. The radiant heat loss (exposure) to a clear sky can allow freezing with the air temp just a bit above freezing. The leaves on top "shade" the bottom material from the radiant heat loss.
Next clear still night stand and face the face and try to feel how cold your cheeks are. Then go stand in a car port where the air is the same temp but face the roof. You can usually feel the difference easily. If not grab your flir camera and it will show you.
If there is much wind the air movement will keep the plants from freezing from exposure.