ADC differential dynamic range vs Differential input voltage range
Either differential input can reside between 0V and AVcc (without potential chip damage) but, if you exceed a difference voltage greater than 2.2 volts then your digital result is out of range.

You should also note figure 32 in the data sheet that gives you an indication of the input circuitry and informs you where the 1 kohm differential input resistance comes from.

Theory of operation (page 17) also informs you that the inputs are each centred at about 2.4 volts with the expectation being that an individual input may impose up to +/-0.55 volts (1.1 Vp-p). Clearly both together give you a capability of 2.2 Vp-p as previously mentioned.

You should also note that gain error could make the 2.2 V typical value for full scale +/-10% different. Zero error can add or subtract +/-10 mV also so, if you expect to use it without calibration checks take into account the potential reduction in range to less than 2 Vp-p.