Effectively each ADC step corresponds to 3.2mV, but the sensors have onboard pre-amplification and signal conditioning circuitry . The EMG and ECG sensors are differential and bipolar, in which the reference voltage is the Vcc/2 = 1.65V midpoint voltage.
The amplification gain depends on the sensor; the EMG has 1000, whereas the ECG has 1100. The sensor always outputs a signal between 0-3.3V, but you have to take into account the gain and midpoint voltage for the conversion.
The BITalino Team