Hi Dneise,
According to the datasheet specs, LSB is equal to VDD/4096, so you are right that it should be around 1.22mV.
There are 2 possible elements that you can look into:
First, what did you use for your analog input signal? Have you checked if this is properly terminated? This might be the reason that you are getting the codes twice as much as your input.
Second, please check if you have a signal conditioning circuit for your board. If there is one, please check if this is in a unity gain setting so we can make sure that the input in the ADC is not yet doubled.
Regards,
May
