Related to differential mode filter characteristic, the 3.3 nF capacitor could be replaced by doubled common mode capacitors. The common mode capacitors are essential, because they also absorb common mode switch charge sourced by the ADC input which would otherwise overload the buffer output.
A practical reason why a separate differential mode capacitor is used can be capacitor ESL and respective self resonance frequency of the input filter. The latter will be shifted upwards by the additional capacitor. It may be also the case that the buffer doesn't well tolerate the larger common mode capacitive load.