How to distinguish between voltage resolution and bit resolution
The 1.22mV is the step size, sometimes also called LSB. Take a look at this google search.

Step size:
Step size is the minimum change in input voltage which can be resolved
by the ADC. The  concept of step size is closely associated with the
resolution of ADC.
Resolution:
The resolution of an ADC refers to the number of bits in the digital
output code of the ADC.

The relation between step size, resolution, and input range can be given by: