Something to do with physics. It’s not just about the heat output from a gas boiler vs heat pump. It’s the output from the radiator to the room that matters. For the same output, a gas boiler heats the water to a higher temperature than a heat pump. Which means a radiator gives out more heat to the room. As an extreme example, if it is freezing outside and the heat pump produces a lot of water at 15c, it may have a high thermal output but still wont keep your room warm. If it produces water at 30c, the radiator will transfer some heat to the room, but unless the radiator is very big or the room is well insulated, probably not enough.