du shows more used space than available space showed by df

Question

I have a remote linux server on which I'm seeing the following weird behaviour:

When I use du -ch /somedrive to get the total occupied space, I get 16TB.
But when I use df -h /somedrive to get the total size of space, I get 10TB, and 500GB still available.

How is the total drive size (by df: 10TB) less than the total occupied space (by du: 16TB)?

---

Here is what the output of df -Th /somedrive looks like:

Filesystems         Type   Size   Used   Avail   Use%   Mounted on
/mapr/somedrive     nfs    10TB   9.5TB  500T    95%    /somedrive

---

Here is what the output of df -Th looks like:

Filesystems         Type   Size   Used   Avail   Use%   Mounted on
/mapr/somedrive     nfs    1.9P   1.4P   560T    72%    /somedrive