By default, the earbuds volume syncs with the device volume. The Xiaomi Buds 5 support for absolute volume refers to the feature where the earbuds' volume synchronizes with the volume level of the connected device by default. This means adjusting the volume on your phone or other connected devices will also adjust the volume of the earbuds accordingly.
If you wish to enable it, you can go to [Settings] >> [Bluetooth] >> [Xiaomi Buds 5] >> [Audio] >> enable [Absolute volume] to keep the volume of the earbuds and your phone strictly consistent.