When you add a new disk and mount and format it, about 5% is reserved for root. So if you have a big disk that could be quite the amount of space “lost”.  The general explanation seems to be that fragmentation isn’t handled very well when the disk is almost full. If you would execute a process doing a lot of file open/close operation it would be very slow. 
Also, some important root processes could be using this space. But, depending on what you are using a disk for  it would not be that important. On all my backup disks for example I have removed the reserved disk space as I will do a better use of it than the OS.

To remove the reserved disk-space just use the following command:  
Use “df -Th” to check how much space you have before and after and you’ll see the difference.

 tune2fs -m 0 /dev/sda1

Cheers
/jima