ubuntu report wrong disk usage on /

On my ubuntu 11.10, /dev/sda3 (150GB) is mounted on / and /dev/sda1 (80GB) is mounted on /home. My entire disk has 250GB and the system is reporting I am running out of disk space. Here is the output of df -h:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             149G  141G     0 100% /
udev                  3.9G  4.0K  3.9G   1% /dev

tmpfs                 1.6G  860K  1.6G   1% /run
none                  5.0M     0  5.0M   0% /run/lock
none                  3.9G  508K  3.9G   1% /run/shm
/dev/sda1              74G   15G   56G  21% /home

Ubuntu disk usage analyzer report the same thing. But this is weird because I believe I never used that much space on /. I also checked every individual directory under / ( exclude /home ), and they don't use that much space:

8.8M    bin
4.0K    dev

0       initrd.img
0       initrd.img.old
828M    lib
15M     lib32
4.0K    lib64
4.0K    media
0       proc
9.2M    sbin
0       sys
1.1G    var

108M    boot
18M     etc
4.0K    mnt
40K     root
4.0K    selinux
72K     tmp
0       vmlinuz
0       vmlinuz.old
16K     lost+found
133M    opt

1004K   run
4.0K    srv
5.9G    usr      

Any one could give me some ideas what is using up the space? Thanks in advance.