Why put a higher BW then "necessary" inside the ADC IC? asked by Nan Z.
A higher BW is needed within the ADC so that an accurate sample can be taken and resolved. If the BW was lower or on the edge or you run an IF up near the edge of the ADC's BW then the internal track-and-hold circuitry will not settle properly. This causes mismatches on the internal frontend giving rise to varying AC performance from part to part.