It is definitely related to it being a deep ground loop geothermal setup. An air-to-air heat pump is trying to get heat out of air that has less heat as it gets colder. A ground loop heat pump is trying to get heat out of water that is not affected by the temperature of the outside air.
With an air-to-air heat pump, you need a backup system for when it gets too cold out for the heat pump to pull enough heat out of the air, and for power outages. With a ground loop heat pump, you only need a backup system for when the power goes out because the ground water will never be too cold for the heat pump to pull heat out of.
With an air-to-air heat pump, you need a backup system for when it gets too cold out for the heat pump to pull enough heat out of the air, and for power outages. With a ground loop heat pump, you only need a backup system for when the power goes out because the ground water will never be too cold for the heat pump to pull heat out of.