black level calibration in image sensor
The thing is that you want on one hand to make the best calibration you can, but on the other hand you don't want to insert nonlinearity or othe artifacts. So you have some analog compensation, in order not to amplify anything that is not actual signal. But still after amplification some offset may remain, so you have to calibrate digitqlly too. 

And you are right, the dynamic range does not span over the full ADC scale. Still, if you have like 90% of your scale it's fine. Hope linear calibration will be enough. I once had a sensor that required polynomial calibration, and even with that the performance was not perfect.