Inconsistency between "du -sh" and "df -h"

I am running a server with Debian stable.

If I call:

df -h

this is the result I get:

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/my-var   2.8G  2.3G  358M  87% /var

While if I call:

du -sh /var

This is the result I get:

832M    /var

How can this happen? Which one is correct? Thank you!