This is due to significant offset in voltage measurement function on this device. It is supposed to be 3.65V for code 64 with 1LSB=57.1mV if the device has no offset in measuring input voltage. If you ‘sum’ input voltage equivalent for both 64LSB code and 128LSB code then that ‘sum’ will more or less will be equal to 64+128=192LSB equivalent on the input. User should use an offset of 0.7V to correct the voltage reading.