This page at energy.gov says the following about geothermal (or ground-source) heat pumps.
Geothermal heat pumps (GHPs), sometimes referred to as GeoExchange, earth-coupled, ground- source, or water-source heat pumps, have been in use since the late 1940s. They use the constant temperature of the earth as the exchange medium instead of the outside air temperature. This allows the system to reach fairly high efficiencies (300% to 600%) on the coldest winter nights, compared to 175% to 250% for air-source heat pumps on cool days.
In normal parlance (as in the fields of physics or engineering), a device's efficiency cannot be greater than 100%. Therefore, I am at a loss to understand what energy.gov is trying to tell me here. What do the numbers mean?