What can I say? I did sleep in my Megalite, with midweight longjohns, heavy socks, and a balaclava, this last January, in Ohio. My guess is that conditions other than actual temperature might factor in: the night was still, in a sheltered campsite in the woods, with no precipitation. Might your other conditions have been a bit harsher, such that wind chill or other factors might make your 20-degree night?

Also, what kind of pad did you use? I was on a BA Insulated Air Core, with a Zrest backup (I forget if I used it on top or or under the mattress.)

Also, I don't sleep particularly cold - though I did start to get a bit chilly around 6am.

Now, I would point out this was not an intentional test. The predicted low was 25 - but predictions have a way of going bad. Afterward, I added lightweight down pants and hooded down jacket to my winter pack; I'm not eager to repeat the experiment.