Could be a difference in the number of windows, or double hung (double leakage) vs single hung (single leakage).
I personally would rather have my house relatively air tight than super insulated & leaking like a sieve.
When i am outside in the cold,it is much harder to stay warm when the wind is blowing at 50 mph than when it is totally wind-still ,even tho the temp is the same in both examples.