Most lightweight tents are mesh inners, with the fly being the weatherproof part. This eliminates the need to vent the tent (in the higher end models anyway). That being said, Roarmeister is correct; the temp difference is mostly due to the wind being eliminated by the nylon tent barrier. With a tarp, you do not have this. Even pitching it side into the wind, there is still convection happening; hence, the need for quality bedding & sleeping bags, as these are what keep you warm. Losing heat to the wind, or the ground, is more prevalent in a tarp than a tent (well, to the wind anyway-ground is still dependent on what you are sleeping on, for the most part).
My setup 95% of the time is a silnylon tarp, with my HH exped hammock, the underpad, and an AMK heatsheet. The heatsheet goes under the foam pad, which goes under me. It provides some radiant heat reflection, as well as being a windproof barrier underneath me. The setup hasnt failed me yet. The best part about having the tarp is that I am not concerned about spilling food on the floor when its pouring out . I live in a no bear area, so, bears arent a concern!