why the used-space of "df -h" is larger than the result of "du -hs /"?

why the used-space of "df -h" is larger than the result of "du -hs /"?

the result of "df -h" shows below:
/dev/sda1       5.6G  3.2G  2.2G   60% /
udev            992M  4.0K  992M    1% /dev
tmpfs           401M  752K  401M    1% /run
none            5.0M     0  5.0M    0% /run/lock
none           1002M     0 1002M    0% /run/shm
the result of "du -hs /" shows below:
1.7G    /
Why is that?