Can anyone explain why a two stage GSHP uses less electricity in cooling mode than in heating mode? 

Because the unit normally runs in first stage while in the cooling mode. In the heating mode the unit runs in first and second stage, plus aux electric strips if installed. Also, the northern third of the country is heating dominate. That means it generally takes twice the energy to heat your home as to cool it.

Bergy

The simple answer: For the same heat transfer rate the delta-T between the earth temp and the cooling/heating coil output is going to be much smaller in cooling mode than in heating mode, therefore more efficient because it's moving the heat up a lower tempererature hill.

You'll always use fewer kwh/CDD (cooling degree-day) to produce 60F air out the coil using 50-55F subsoil in cooling mode than the kwh/HDD (heating degree-day) you use to produce a 90F air stream out of 50-55F subsoil (or colder as the season goes longer) in heating mode. To pump the heat up a bigger temperature difference takes more energy. If you have 85F subsoil, it would flip the other way- cooling would become more expensive than heating. If your subsoil was 70-75F it would run about the same in either mode.

If you are in a heating dominated climate, this is especially true if auxiliary heat is employed.
If you are in a cooling dominated climate your observation may not be correct.
Joe

Agree with Dana1 except that heating if soil temp is 70-75 will be more efficient since you also recover the heat from the compressor in heat mode while you have to offset that additional energy in the cooling mode.