The point lies in the way the values are computed. These values are seen to arise out of a calorimetry experiment.
The Gross Calrific Value (GCV) is calculated in such a way that the products after combustion of a particular material are considered to be in their pre-combusted state. For example, water molecules formed after combustion are considered to be in the physical state that corresponds to pre-combustion conditions (The products formed after combustion are treated as what they would be at 15 deg C, so water will be a liquid.). So, any calculation of GCV, of any system, will assume that water is always formed in the liquid state. This will take care of any phase changes that has happened. That is, if water vapours are formed, it will be clearly seen in the experimental data as additional latent heat of vaporisation. The energy used up to convert water into vapors during the combustion process will be obtained here.
However, Net Calorific Value (NCV) will do no such assumption. The energy used up for phase changes like this are not taken into consideration. It means that the energy spent for such phase conversions cannot be recovered.
So, GCV will have higher valus than NCV.