A shift in the data read from the FIFO could be due to sub-DAC XNL error mentioned in the data sheet. The sub-DAC tag identifies that this occurred. The data sheet says: “One application of this feature would be to trigger special backend software handling for the conversions on which the sub-DAC update occurs to compensate for the residual error.” This means the microprocessor looks for this tag in the FIFO, and if present, the microprocessor then either corrects the error (or throws it out in favor of the next reading).