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I often see differential amplifiers/ADCs with a conditioning circuit that looks like this:

I'm trying to figure out how Cdiff helps reduce noise/improve things. So far as I can figure out it's basically there to help ease the tolerance mismatch on the two common mode capacitors. In other words suppose that the two common mode caps have a tolerance of +/-20% -- that could ostensibly lead to a worst case scenario (C1 = .8C and C2 = 1.2C) with a large difference in cutoff frequencies for the common mode filters. This would mean that for a certain range of frequencies (e.g. those between the two cutoff frequencies in particular) common mode nose at those frequencies would appear as differential noise. Is the differential cap there to effectively reduce this effect? If the differential cap is much larger than the common mode caps then all of a sudden the tolerances on the common mode caps would seem to have a reduced effect.

Is that it? Is there any other way that differential capacitor helps reduce noise?

In my experience with several ADCs, the ADC can take a surge of current when initially calculating a sample. This can modify the voltage at the input i.e. there is the possibility of a measurement error.

These types of ADCs are referred to as charge-redistribution successive-approximation devices or sometimes switched-capacitor ADCs: -

Analog Devices source here.

Adding a capacitor at the input means that this surge of current does not significantly "alter" the voltage being measured because the external capacitor chosen at the input is many times the value of the internal capacitors.

The AD source above describes how the effect can also compromise the reference voltage and this, of course, is also relevant to protecting the integrity of the input signal.

I'm trying to figure out how Cdiff helps reduce noise/improve things.

It also does what you say in your question i.e. it will reduce mismatch between the two CM capacitors but I would say that this is secondary to its main purpose.

CM 20% mismatch resulting in ~ 4dB less attenuation of CM noise is not too significant as the potential to rise in DM impedance from ground path ESL and ESR effects for caps in the mOhm impedance range.

The CM cap does both CM and DM attenuation but the series input R’s in the ADC and transient switched cap loads, the diff cap does a better job to maintain the SRF of the caps.

Any nH rise ESL in the CM path of CM caps also lowers the self resonant frequency of those caps to attenuate the step load capacitance from ultra high speed C switches in the cap switched SAR ADC.