why my file systems shows -71G and how can i fix it

Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ada0p2      1G    624M    302M    67%    /
devfs          1.0k    1.0k      0B   100%    /dev
/dev/ada0p4    503M     20k    463M     0%    /tmp
/dev/ada0p5    9.9G    1.7G    7.3G    19%    /var
/dev/ada0p6    903G    903G    -71G   109%    /usr
devfs          1.0k    1.0k      0B   100%    /var/named/dev