That's why there is a position feedback potentiometer inside the analog servo to tell the amplifier "where" the servo "is" at any time. It can then calculate how much power(voltage) to apply to get back to where it belongs. The further off it is, the more power it applies. As I said earlier, the analog applies a variable voltage. The digital is more like a pulsed on/off switch that switches (at a very high speed) the full voltage.