I’m also pretty certain the loudness measurements are inaccurate if you’re not using Apple headphones or the Apple dongle. How would it be able to know the sensitivity of every headphone’s driver via analog input? Even if it could detect the headphone’s impedance, the sensitivity rating is more prudent to determining overall volume level, and that rating is variable depending on the headphone model, and can even vary depending on the content you’re listing to.

Also, if you’re connecting your headphones wirelessly, you can often adjust the volume on both the phone, and the external receiver, separately. In this case, the phone volume should be set to max and you can adjust the overall volume from the Bluetooth device itself. In these common cases, the phone can’t accurately judge your headphone’s true volume level because it’s not controlling the level at all.