Why Are Some Volume Levels Linked?

The Ringer, System and Notification volumes are linked on most new devices. It's an Android OS limitation. 

There is a work-around with the Separate ringer and notification feature.

You can enable it from the Settings screen of the app.

You can navigate to the settings screen from the little menu (3 vertical dots) to the top right of the app.

The Separate ringer and notification feature is a work-around that uses the Ringer volume in Volume Control for phone calls.

Incoming calls will have a separate volume level from Notification sounds.

Here's how it works:

When a call comes in, Volume Control will change the volume to your custom Ringer volume level. It reverts back to the old levels after the phone call.