CMOS switches such as the DG419 have a threshold voltage - i.e. the voltage at which the output state is flipped - which is dependent upon the logic level and the positive supply rail. However, the exact threshold voltage at which the state will flip is undefined. For the DG419, the threshold occurs within a region spanning many volts, particularly for lower supply voltages:
What causes this uncertain region in switching? I guess it's not the material itself, but rather something else such as stray capacitance or source impedance.