15 GB of unaccounted-for space in filesystem

[nathanb /mnt/work] sudo du -hs .
23G .
[nathanb /mnt/work] df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb1        40G   38G  6.4M 100% /mnt/work

Where is the other 15 GB?

/dev/sdb1 on /mnt/work type ext4 (rw,nosuid,nodev,relatime,data=ordered)