Somewhere in the history of this external storage device, two volumes were created on the device, and both volumes are now mounted, and given you’ve just asked macOS to dismount (eject) a volume), macOS is now asking whether you want to eject just the specified volume, or whether you want to dismount (eject) everything.
If you’re intending to remove the external storage device, you’ll want to dismount (eject) all volumes.
It’s possible there’s a second partition present, but this is probably a volume created by time Machine.
Time Machine volumes are usually left attached, too. Unless you’re moving the system to another location, or maybe cycling multiple Time Machine backups.