UBIFS space consumption

[Vikram Narayanan <vikram186@gmail.com>]

I am using UBIFS with the below partitioning layout.

<6m(bootloader)
512k(environment)
512k(redundant-environment)
5m(kernel),
170m(filesystem),
40m(user_data1),
-(user_data2)>

For the UBI volume 0 which has 170 MiB,
ubinfo reports 164.7 MiB, which is explained by:

(PEB size - LEB size) * (no of PEB's - reserved PEBs)
(256 - 248) * (680 - 6) = 5.392 MiB
Total size = 170 MiB - 5.39 = 164.6 MiB


root@BOX:/# ubinfo /dev/ubi0
ubi0
Volumes count:                           1
Logical eraseblock size:                 253952 bytes, 248.0 KiB
Total amount of logical eraseblocks:     680 (172687360 bytes, 164.7 MiB)
Amount of available logical eraseblocks: 0 (0 bytes)
Maximum count of volumes                 128
Count of bad physical eraseblocks:       0
Count of reserved physical eraseblocks:  6
Current maximum erase counter value:     5
Minimum input/output unit size:          4096 bytes
Character device major/minor:            253:0
Present volumes:                         0

But according to "df",  UBI volume 0 is sized 148.9 MiB. That's, 15.8 
MiB more in terms of consumption.

root@BOX:/# df -h
Filesystem                Size      Used Available Use% Mounted on
ubi0:rootfs             148.9M    132.8M     16.1M  89% /
root@BOX:/#

Can someone explain where this 15 MiB is getting lost?

Regards,
Vikram