The ADC module has been calibrated during the production of the device. This calibration value is stored in the production signature row of the device. The calibration value compensates for the mismatch between the individual steps of the ADC pipeline and it improves the linearity of the ADC.
The calibration value is not loaded automatically, and should always be loaded from the production signature row (ADC x CAL0/1) and written to the corresponding ADC calibration registers (CALL/CALH) before enabling the ADC. Flowcharts for loading stored calibration settings are shown in figure "Using Stored Calibration Settings".
The calibration value is factory calibrated with high accuracy equipment to the data sheet accuracy, and is not intended for user calibration.
The application note “AVR120: Characterization and Calibration of the ADC on an AVR” contains more information on characteristics of ADCs and how to compensate for gain and offset errors.