df -h, du -ahx and flash size not match

[Esteban Lopez <elopez@softel.net.mx>]

Hello List

I have a linux box with debian 5, kernel 2.6.27, running over a nand 
flash with two partitions

cat /proc/mtd
dev:    size   erasesize  name
mtd0: 04000000 00020000 "Partition 1"
mtd1: 1c000000 00020000 "Partition 2"

The Partition 2 is my ROOTFS where I have ubifs. The size of this 
partition is 0x1c000000=469762048 bytes


My ubinfo output is this

ubinfo -a
UBI version:                    1
Count of UBI devices:           1
UBI control device major/minor: 10:63
Present UBI devices:            ubi0

ubi0:
Volumes count:                           1
Logical eraseblock size:                 129024
Total amount of logical eraseblocks:     3583 (462292992 bytes, 440.9 MiB)
Amount of available logical eraseblocks: 0 (0 bytes)
Maximum count of volumes                 128
Count of bad physical eraseblocks:       1
Count of reserved physical eraseblocks:  35
Current maximum erase counter value:     33
Minimum input/output unit size:          2048 bytes
Character device major/minor:            253:0
Present volumes:                         0

Volume ID:   0 (on ubi0)
Type:        dynamic
Alignment:   1
Size:        3544 LEBs (457261056 bytes, 436.1 MiB)
State:       OK
Name:        rootfs
Character device major/minor: 253:1


Like you can see I have 3544 LEBS to work = 457261056 bytes

Well my question is that if I run a df -h commnad I get this output
df -h
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                  30M     0   30M   0% /lib/init/rw
udev                   10M   48K   10M   1% /dev
tmpfs                  30M  4.0K   30M   1% /dev/shm
rootfs                236M  161M   75M  69% /
/tmp                   30M     0   30M   0% /tmp

Why only 236M???


If I create big files and I run du -ahx command over / I get a  total of 
593M
  of info
Why the size not match ? why I can have 593M in my ROOTFS if I only have 
469762048 bytes?? Why df -h report 161M used in rootfs if du report 593M?

Could you help me to understand this?

Thank you !!!