The total error of the actual ADC comes from more than just quantization error. This document describes offset and gain errors and how to compensate for them. It also describes two measures for non-linearity; differential and integral non-linearity.
For most applications, the ADC needs no calibration when using single ended conversion. The typical accuracy is 1-2 LSB, and it is often neither necessary nor practical to calibrate for better accuracies.
However, when using differential conversion the situation changes, especially with high gain settings. Minor process variations are scaled with the gain stage and give large parameter differences from part to part. The uncompensated error is typically above 20 LSB. These variations must be characterized for every device and compensated for in software.
At first sight 20 LSB seems to be a large value, but does not mean that differential measurements are impractical to use. Using simple calibration algorithms, accuracies of typically 1-2 LSB can be achieved.