What causes the uncertainty in the threshold voltage of a CMOS switch?
It's a CMOS input, so there are two transistors 'fighting' each other at the input threshold. 

Variations in the semiconductor manufacturing process will result in different behaviors for one vs. the other, so the point where they switch will vary. That variation is accentuated at relatively low supply voltages where the transistors are barely working at all (what is 'relatively low' will be different for a high voltage analog switch like the DGxx series vs. a chip designed for 3.3V Vdd). 

Chances are that under fixed and benign conditions (temperature and supply voltage) the threshold will stay pretty much fixed for any given sample of chip.