IIRC, many of these digital attenuator ICs do two things for volume control, a voltage divider on the input and switched gain resistors for the following opamp. You want an output voltage below that of the input, the opamp runs at unity gain and the voltage on the input is divided down. You want greater than the input voltage, it starts to increase the gain of the opamp. Now, I have absolutely no idea what IC Justin is using here, so it is definitely possible it could be working different, but this seems a likely configuration.