LTSpice - varistor component
If you read in the LTspice manual, you'll see that it says it's breakdown voltage is set by the voltage applied at its control terminals. The voltage can be of any sign, any polarity (it counts as abs(V(ctl))), and the calculation is like this: if your applied voltage is 10V, the series resistance is 1 Ohm, Rclamp=1 and the control voltage is 1V (I presume), then the varistor will breakdown at 1V (the controlling voltage), meaning from the total of 10V you'll have 10-1=9V which, divided by the total resistance of 3 Ohms results in 3A. If your source voltage is 7V, series resistance is 2 Ohms, Rclamp=1.5 and controlling voltage 3V => (7-3)/(2.5+1.5)=4/4=1A.