UFS keeps extra space that is not available to regular users so when the filesystem is completely filled for user and non-user data, it shows 108%.
: tunefs -p /
tunefs: POSIX.1e ACLs: (-a) disabled
tunefs: NFSv4 ACLs: (-N) disabled
tunefs: MAC multilabel: (-l) disabled
tunefs: soft updates: (-n) enabled
tunefs: soft update journaling: (-j) enabled
tunefs: gjournal: (-J) disabled
tunefs: trim: (-t) disabled
tunefs: maximum blocks per file in a cylinder group: (-e) 4096
tunefs: average file size: (-f) 16384
tunefs: average number of files in a directory: (-s) 64
tunefs: minimum percentage of free space: (-m) 8%
tunefs: space to hold for metadata blocks: (-k) 6408
tunefs: optimization preference: (-o) time
tunefs: volume label: (-L)
Note the value for -m