Ok, when you create a partition, there is some space that is allocated to root some % don't think it goes beyond 10%. You see negative value because you are using part of that space. You can use
and get to the bottom of it, also read the man page of tunefs look for
Specify the percentage of space held back from normal users; the
minimum free space threshold. The default value used is 8%.
Note that lowering the threshold can adversely affect performance
but be carefull from here on, don't have to remind you to back up, coz with great power comes great responsibility.