Why df totals don't come close to adding up

Linux filesystems routinely reserve between 1 and 10 percent of the filesystem for the root user. This is to ensure the root user can function when a non-root user "fills" the filesystem. The df command does not show this reserved space as available.

As root, run this command: /sbin/tune2fs -l /dev/sda1
Look for the line that reads "Reserved block count".

You can also use tune2fs to change the amount reserved.