To me it seems that:
1). The temperature in the tent is equal to the temperature outside when it is set up
2). Temperature is much higher when people have stayed there for some time
3). Temperature gets lower if it it windy and wind accesses (just a little bit) the insides of the tent
4). Subjectively (and maybe this is where I'm wrong) it seems that 2 layers create a less penetrable wind protection - just because.