A cap tube is primarily a pressure-reducing device. When either gas or liquid is used, the reduction in pressure is linear (the pressure is reduced at the same rate for every measured length of the cap tube).
If liquid refrigerant responded the same way, it could never be used as a refrigerant control. If the inlet pressure was high, the evaporator would be flooded. If the inlet pressure was low, the evaporator would be short of refrigerant.