Heat pumps move heat. In the summer, it’s pulling heat from inside and moving it outside and the opposite of that in the winter.

Basically, the temperature differential is what makes the difference. The larger the differential, the more energy it has to use.

In the winter, when it’s 30 degrees (F) outside, and you want it to be 70 inside, that’s 40 degrees it has to move. In the summer when it’s 90 degrees outside, and you want it at 70, that’s only 30 degrees.

Air source heat pumps, as the name implies, pull heat from (and exhaust heat to) the ambient air. When it’s really cold in the winter, there’s less ambient heat to move inside, so it has to run longer. Some (all?) heat pumps also have an auxiliary resistive heating element to make up the difference which lowers efficiency quite a bit.

Granted, newer heat pumps can work well down to lower temperatures than the older ones I’m familiar with, but in a nutshell, that’s why they can potentially use less energy in the summer.