Precision and accuracy are different.
https://en.wikipedia.org/wiki/Accuracy_and_precision
24-bit Sigma-delta converters are precise to 24 bits but are less accurate.
For the given case., it shifts the transfer curve by that much implies that it's range is changed. So you don't get accurate results., but it doesn't matter in some applications. In voice or xDSL applications the absolute accuracy of the ADC is even not a key feature.
Rarely you'll see some sigma delta ADC datasheets that can match accuracy with resolution but they wont be more than 16-bits.
//Copy pasted from the white paper **broken link removed** //
Resolution and Accuracy
One of the more confusing tasks when evaluating data sheets is to separate the accuracy
of an ADC from its resolution. Just because an ADC exhibits 16 bits of resolution does
not guarantee 16 bit accuracy. In fact, an ADC may not even maintain monotonicity (each
bit combination measuring a higher value than the next lower combination) over its full
operating range, especially over the temperature range. In some applications, resolution
is more important than accuracy; it may be more important to be able to resolve small
changes in a variable than to know the absolute value of the new signal level. Many 16-bit
resolution ADCs exhibit 14 bits of accuracy, and this is sufficient for a large number of
applications.
In some applications absolute accuracy is paramount; therefore overall accuracy becomes
more important than resolution. In these cases, it is important to look at the integral
nonlinearity specification. This represents an error which cannot, in general, be eliminated
with calibration.
Understanding the overall system requirements will aid in the selection of the appropriate
ADC without overspecifying its characteristics. Higher cost is generally associated
with ADCs that have more stringent characteristics. If dynamic accuracy is important, then
it may be better to choose a faster, lower-resolution ADC than one with high resolution
but longer conversion time.