Strange Error

Hi,

A strange error is happening in my server.
I run de df command :

Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda3 68437272 46542632 18418176 72% /
/dev/sda1 101089 14922 80948 16% /boot
none 512696 0 512696 0% /dev/shm
/dev/tmpMnt 984248 284080 650168 31% /tmp
/tmp 984248 284080 650168 31% /var/tmp

My tmp = 31% (NEVER CHANGE)

But when I access my /tmp and I execute the command du:

root@ns1 [/tmp]# du -h
4.0K ./.ICE-unix
4.0K ./.font-unix
1.5M .

My /tmp shows only 1.5M full.

Why?
This problem began after a reboot.
My fstab:

/dev/sda3 / ext3 defaults,usrquota 1 1
/dev/sda1 /boot ext3 defaults 1 2
none /dev/pts devpts gid=5,mode=620 0 0
none /proc proc defaults 0 0
none /dev/shm tmpfs defaults 0 0
/dev/sda2 swap swap defaults 0 0
/dev/fd0 /mnt/floppy auto noauto,owner,kudzu 0 0
/dev/cdrom /mnt/cdrom udf,iso9660 noauto,owner,kudzu,ro 0 0
/dev/tmpMnt /tmp ext2 loop,noexec,nosuid,rw 0 0