It is called thermal inertia from the structure and the ground. If you have a masonry home it would be 10F higher.
The lighter the construction materials, the faster the temperature drop until the indoor temp gets low enough. Most codes do not recognize the thermal mass storage of a structure and rely on short term lab R-value rating as a measure.
My all masonry home in northern Minnesota never got low enough to kick on the heat (set at 55F) until December, when we were already below zero many times.