Variations in the manufacturing process is not abnormal, and results in inconsistencies between different individual ADCs. As each die on a wafer is uniquely fabricated, the performance between two ADCs is never exactly the same, only behaving similarly. Some examples where this shows across devices is in gain error, linearity behavior, and reference voltage drift. Each ADC can have slightly different package or manufacturing parasitics as well, all of which are resolved through calibration.