I guess the term most are elluding to but not exactly putting their finger on it is << MICROCLIMATE >>
With the net on, the moisture in the air inside the net will allow the air to retain a little more heat.
Think of a net run into the water, yes the water will go right thru, but if the holes are small enough, the water will "clog" the holes until you shake the net. This will create a small microclimate that will allow a little heat retention. Just like the closed tarp will also help.