Hooked up a PCM5102 board to a Teensy 3.1 to form a USB audio interface.

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With the code below, my windows PC sees a USB Teensy Audio device and outputs audio via the PCM5102 board as expected.
// PCM5102 bd   Teensy
// VCC          5V
// GND          GND
// LRCK         23
// DATA         22
// BCK          9

// set Tools > USB Type to Audio (+ Serial if needed)

#include <Audio.h>

AudioInputUSB            usb1;
AudioAmplifier           amp2;
AudioAmplifier           amp1;
AudioOutputI2S           i2s1;
AudioConnection          patchCord1(usb1, 0, amp1, 0);
AudioConnection          patchCord2(usb1, 1, amp2, 0);
AudioConnection          patchCord3(amp2, 0, i2s1, 1);
AudioConnection          patchCord4(amp1, 0, i2s1, 0);

void setup()
  digitalWrite(LED_BUILTIN, HIGH);

void loop()
  float vol = usb1.volume();  // read PC volume setting
  amp1.gain(vol);             // set gain according to PC volume
To control the volume of the audio I use the windows volume slider:

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Name:	Windows audio volume 50%.png 
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Now the interesting part is that when the slider is at 50%, the value of variable "vol" = 0.63. I expected 0.50.
When I checked other slider settings, I see the following values for slider percentages:

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My questions now are: where [and how] is this conversion done? And is there a reason it isn't linear?
I have been digging through the audio library sources but could not find it.

Any help and/or explanation is appreciated!