Difference Of Used Space Between Du -hs And Df

I have a partition /dev/sda2 which is mounted to /webroot.

Today, I find that I can't upload any files via IPB (running on lighttpd)
So, as usual, I went on SSH and tried df in order to see how much space is left on the server:

Filesystem 1K-blocks Used Available Use% Mounted on
<deleted>
/dev/sda2 20641788 19483292 109856 100% /webroot
Since lighttpd is running inside chroot, I thought it was the log files but even after I deleted them, it is still at 100% usage.
After spending some time, I couldn't find any large files so I did du -hs to see which folder is using large amount of space.

web1:/webroot# du -hs /webroot
2.2G /webroot
I'm without a clue on this as what du -hs is telling me and df is different but it seems like the partition is really full.

Any ideas?

Here is the output of tune2fs for reference:

web1:/webroot/var/run/lighttpd# tune2fs -l /dev/sda2
tune2fs 1.40-WIP (14-Nov-2006)
Filesystem volume name: /webroot
Last mounted on: <not available>
Filesystem UUID: 62625b28-9ad6-4258-8acc-0bfa0e7b5f48
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal filetype needs_recovery sparse_super large _file
Default mount options: (none)
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 2626560
Block count: 5242880
Reserved block count: 262160
Free blocks: 5123494
Free inodes: 2626465
First block: 0
Block size: 4096
Fragment size: 4096
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 16416
Inode blocks per group: 513
Filesystem created: Fri Jun 27 13:34:27 2008
Last mount time: Sun Jun 29 01:11:27 2008
Last write time: Sun Jun 29 01:11:27 2008
Mount count: 7
Maximum mount count: 36
Last checked: Fri Jun 27 13:34:27 2008
Check interval: 15552000 (6 months)
Next check after: Wed Dec 24 12:34:27 2008
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 128
Journal inode: 8
First orphan inode: 820811
Default directory hash: tea
Directory Hash Seed: de38e2ce-4ee2-4ecd-a562-16ecb5982500
Journal backup: inode blocks
Also /etc/fstab entry for /dev/sda2:

web1:/webroot# cat /etc/fstab|grep sda2
/dev/sda2 /webroot ext3 defaults 1 2