I'm guessing Ubuntu always wants to use subvolumes, if you use btrfs for the root partition you will automatically get two subvolumes, "@/" and "@home/" (see the Ubuntu wiki btrfs entry for more info on this), and it might set up the "@home/" subvolume just for the sake of consistency.
On a side note: If you look in /etc/fstab you should see the option "subvol=@home" for the /home mountpoint, which means that it mounts the "@home/" subvolume of that btrfs (subvolumes are quite similar to directories in some sense).
Answer to comment:
Yes, you can mount the top node and the subvolume node simultaneously:
mount /dev/sdX# /mnt/top_node -o subvolid=5
mount /dev/sdX# /mnt/home_subvol -o subvol=@home
(If the top node is the default mount point (default case) you can omit the "-o subvolid=5" option)
Here the @home/ subvolume will be accessible both through /mnt/top_node/@home/ and /mnt/home_subvol/ equivalently.
It might be worth it to have the two dirs there alongside @home/ be subvolumes instead, that way you could snapshot and mount them all independently...