df/du shows big difference of used space

[niclasw <niclaswmail@gmail.com>]

I have a 1500G hard drive, encrypted.
Different commands shows different usage:

As root, from root directory:
df -m
Filesystem           1M-blocks      Used         Available     Use% 
Mounted on
/dev/mapper/d       1390840   1284525    35665          98% /

That is 1284G used of 1391G which would be like 92%!
Not 98%
Why does it say 98%

du -sh *
adds up to about 933G

So the question is why the big difference of used space between df and du

du shows 933G used
df shows 1284G used

Can anyone explain the big difference, 350G, of used disk space between 
du and df commands?

/Niclas