Why would that be since you are extracting heat from in-ground water at roughly 55 degrees, year-round? I would think that the efficiency would be consistent no matter what the air temperature (since it doesn't use outdoor air as the heat exchange medium). I obviously don't understand something about geothermal.
The liquid in our loops is 59 F year round. Our power bill actually drops when we switch to AC mode.
You aren’t really extracting heat from the ground, it is the heat achieved from the change of state of the coolant. The cold is sent to the outside earth in the winter and the heat is retained inside. It really is just a giant refrigerator. That hot coil on the back of the refrigerator is indoors in winter and is switched to the outdoors in summer by changing the direction of coolant flow, making the evaporator inside during the winter and outside during the summer. If the ground were to freeze around the “evaporator” tubes the system will cease functioning. Here in Wisconsin the outside evaporator is 10’ underground and hundreds of feet long. A well designed system is very frugal...many older systems are under-designed from what I hear...
It seems to me you have a pretty good understanding. I suspect that the BTU output of the unit is simply inadequate to provide enough heat to keep the place warm when the temperatures are very cold. Like any furnace, if it has to run all the time, it may not output enough heat to completely warm the house.