Are all the compared heat pumps working at the same temperature levels? Otherwise, the COP isn't a good comparison metric, as even for an ideal heat pump with a COP = 1 / carnot_efficiency, it depends on the temperatures.

For instance, a heat pump lifting heat from 20°C to 100°C with a COP of "only" 2 is excellent, whereas lifting heat from 20°C to 30°C with a cop of 3 is not impressive.