Why does the volume of the Android system sometimes not change synchronously when the volume of the Xiaomi Bluetooth Speaker Mini is adjusted?

The speaker's volume and the Android system's Bluetooth volume are adjusted separately when listening to music. For example, if the speaker has been adjusted to the maximum volume, however, the volume on the mobile phone has not been adjusted to the maximum, the speaker will not reach the maximum volume.
When playing games on a mobile phone, the audio goes through the SCO channel, and the speaker volume is consistent with the phone call volume. When the volume of the speaker is adjusted, the volume on the device will change synchronously.