du doesn't include metadata used to store pointers to data blocks. And depending on the filesystem type, that can be larger/smaller (ext4 is relatively small unless it is converted from ext2/3, btrfs even smaller).
There is also the possibility that the space is being constrained by the space reserved for root (5). This amount can be reduced, as no user can write to /usr (unless it happens to include /usr/tmp which is usually a symbolic link to /tmp). For /usr, this reserve can be set to 0.
For most systems / and /usr are the same filesystem, and are 10G to 20G depending on the intended usage. The more packages installed (such as for a workstation) the more space needed. In the combined / and /usr, usually /tmp is mounted as a tmpfs filesystem (which resides in memory and gets flushed on every boot).
Last edited by jpollard; 03-09-2015 at 12:59 PM.