Dividing 6.144 volts by 32767 yields a scale factor of 0.1875 mV per bit. This is a significant improvement over the Arduino ADC which resolution of approximately 5 mV per bit. In fact, its about 26 times better!
The neat thing about this is that this is worst case. In another PGA setting, you can establish a full scale of +/- 2.048 volts. That provides us a resolution of 0.0635 mV.