Couple of possibilities one being that the port that the SG connects to may not have access to the full WORD of the injector duration i.e. it may be a 16 bit duration but only provide the upper 8 bits to the interface. Also the SG many not have the computing power to handle more resolution in width or speed of its calculations. Also I hear that the fuel pressure is also adjusted via a regulator that is controlled by the ECU so that injector timing is not the only variable as is also the amount of vacuum on the intake affecting the fuel flow from the injectors - higher vacuum would draw in more fuel than low vacuum. Lots of variables - anyong ever notice how the gas pedal response is a little slow and inconsistant? I do.
I used to drive a VW Rabbit with CIS Continuous Injection System which operated with an air cone and plate metering air flow into the engine which connected to an arm regulating the fuel being injected. That setup was continuous ANALOG fuel control and very responsive and by simple modifications to the Air Cone shape you could change the A/F ratio.