Apparent Negative free disk space and FreeBSD 12

Just curious as to what’s happening here. This is just a non-prod VM that I have at home. Curious as to what leads to the ‘negative’ available diskspace. This is FreeBSD 12 running under VMWare ESXi 7.

FreeBSD ssh1.rynhart.co.nz 12.0-RELEASE-p10 FreeBSD 12.0-RELEASE-p10 GENERIC  amd64
[root@ssh1 /fixeddisk]# df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/da0s1a     15G    6.5G    6.9G    48%    /
devfs          1.0K    1.0K      0B   100%    /dev
/dev/da1s1a    901G    861G    -32G   104%    /fixeddisk