Some equipment has a signal common and a ground that are not the same. With separate signal common and ground the volume pot is not grounded. The following is the case for signal common and ground being the same.
The typical volume pot takes all of the input and "trims" some off. For it to dissipate some of the incoming volume, amplitude, the voltage has to be referenced to ground. The input voltages come in from the center of the input jack to ground where the input jacks. What happens is the volume pot is from top to bottom "across" the selected input; meaning the whole resistance of the pot is across the input. The bottom of the volume pot is grounded. The top is attached to the selector switch and directly to the incoming center, hot, conductor. The wiper can go from the bottom, where it is grounded, to the top where it is attached directly to the input voltage. Anywhere in between top and bottom a portion of the input is sent downstream to the rest of the circuitry but not all of it.
