Unexpected value using kAudioDevicePropertyVolumeScalarToDecibels - macos

Question

I'm getting unexpected values for the volume level using kAudioDevicePropertyVolumeScalarToDecibels on my laptop's built-in audio.
void volume_test()
{
AudioObjectPropertyAddress address = {
.mSelector = kAudioHardwarePropertyDefaultOutputDevice,
.mScope = kAudioObjectPropertyScopeGlobal,
.mElement = kAudioObjectPropertyElementMaster
};
AudioObjectID deviceID = kAudioObjectUnknown;
UInt32 dataSize = sizeof(deviceID);
OSStatus result = AudioObjectGetPropertyData(kAudioObjectSystemObject, &address, 0, NULL, &dataSize, &deviceID);
assert(result == noErr);
address.mSelector = kAudioDevicePropertyVolumeScalar;
address.mScope = kAudioObjectPropertyScopeOutput;
Float32 volumeScalar = 0;
dataSize = sizeof(volumeScalar);
result = AudioObjectGetPropertyData(deviceID, &address, 0, NULL, &dataSize, &volumeScalar);
assert(result == noErr);
address.mSelector = kAudioDevicePropertyVolumeDecibels;
Float32 volumeDecibels = 0;
dataSize = sizeof(volumeDecibels);
result = AudioObjectGetPropertyData(deviceID, &address, 0, NULL, &dataSize, &volumeDecibels);
assert(result == noErr);
address.mSelector = kAudioDevicePropertyVolumeScalarToDecibels;
Float32 convertedVolumeDecibels = volumeScalar;
dataSize = sizeof(convertedVolumeDecibels);
result = AudioObjectGetPropertyData(deviceID, &address, 0, NULL, &dataSize, &convertedVolumeDecibels);
assert(result == noErr);
address.mSelector = kAudioDevicePropertyVolumeDecibelsToScalar;
Float32 convertedVolumeScalar = volumeDecibels;
dataSize = sizeof(convertedVolumeScalar);
result = AudioObjectGetPropertyData(deviceID, &address, 0, NULL, &dataSize, &convertedVolumeScalar);
assert(result == noErr);
NSLog(#"Direct = %.4f %+2.2f dB", volumeScalar, volumeDecibels);
NSLog(#"Converted = %.4f %+2.2f dB", convertedVolumeScalar, convertedVolumeDecibels);
address.mSelector = kAudioDevicePropertyVolumeRangeDecibels;
AudioValueRange decibelRange;
dataSize = sizeof(decibelRange);
result = AudioObjectGetPropertyData(deviceID, &address, 0, NULL, &dataSize, &decibelRange);
assert(result == noErr);
NSLog(#"dB range %+2.2f ... %+2.2f", decibelRange.mMinimum, decibelRange.mMaximum);
}
The output is:
Direct = 0.0620 -47.69 dB
Converted = 0.0620 -59.56 dB
dB range -63.50 ... +0.00
The same thing occurs using the underlying AudioControl directly.
For reference, Audio MIDI setup shows:
Interestingly, using my external display's audio and the elements from kAudioDevicePropertyPreferredChannelsForStereo (since it has no master element), the values match.
Also of note, for the display audio kAudioDevicePropertyVolumeDecibelsToScalarTransferFunction is 5, or kAudioLevelControlTranferFunction2Over1. Attempting to retrieve kAudioDevicePropertyVolumeDecibelsToScalarTransferFunction for the laptop fails with the message
HALC_ShellObject::GetPropertyData: call to the proxy failed, Error: 2003332927 (who?)
HALPlugIn::ObjectGetPropertyData: got an error from the plug-in routine, Error: 2003332927 (who?)
which is not a "normal" error message for an unsupported property.
How should kAudioDevicePropertyVolumeScalarToDecibels be used?