Root has a percentage of free space reserved.

Login as root via ssh
Password is recalboxroot

Run df -h
If it shows the size for "Available" as being "0" and "Use" as 100%, but "Used" is < "Size"...

Run dumpe2fs /dev/mmcblk0p3 | grep -i reserved
This will prove the space is reserved.

Now that space was reserved for the root user so to reclaim that space, run
tune2fs -m 0 /dev/mmcblk0p3

Now the root user can run out of space and there are some possible problematic issues that could occur, but you can use your full capacity.

Warning: This should only be performed by power users who are not afraid of the command line and understand the commands they are running.