why the result of df a partition is different from du so much

[haha]

[@4.54 ~]# df -k /var
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/mapper/vgroot-lvvar
4062912 3424616 428584 89% /var
[@4.54 ~]# du -sh /var
120M /var
[@4.54 ~]# lsof /var
[@4.54 ~]#

but after remount /var
the result of df -k /var is almost as same as du

btw:sorry for my poor english:)

[haha]

This problem has been solved
:)
I just execute "/etc/rc.d/init.d/psacct start" once
then "df -h"
the result is as same as du