Compressor maps plot efficiency versus flow and pressure ratio. Pressure ratio is the absolute boost pressure divided by the inlet pressure. Ideally, the inlet would be one atmosphere at sea level or ~14.7psia. However, intake restrictions result in a pressure drop which lowers that value. Because the same boost is divided by a lower inlet pressure, the pressure ratio is higher on the restrictive intake and lower on the dual intake. The dual intake shifts the operating point down the compressor map toward a higher efficiency island. This would allow for higher mass flow at lower temperature with the same boost pressure. There should also be a gain from a simple reduction of pumping losses. Without measuring all of the pressures and temperatures in the intake tract, I couldn't say how much each contributes to the results we are seeing.
