Current mode operation of a Source Measurement Unit (SMU)
It might all become clear if you consider that the function of the differential amplifier is to measure the voltage across Rr and present the same voltage on the output of the differential amplifier with respect to ground.  Using your virtual ground technique, you correctly (and easily) found the output voltage of the differential amplifier.  Since this output voltage (with respect to ground) is the same as the voltage across Rr, the Error Amplifier output will be driven to whatever voltage is necessary to achieve this voltage across Rr. So you are sensing the voltage across sense resistor Rr and adjusting the output to give you a current source which is proportional to Vref.