size discrepancy after tarring a dir

[JD]

  I have two mounted disks,  both ext3 mounted
as
/sdb1
/sdc1

On /sdb1 I have a directory, let's call it dirx.

1. rm -rf /sdc1/dirx

2. cd /sdb1
3. tar cf - dirx | tar -C /sdc1 -xpf -

Neither dir (/sdb1 and /sdc1) are not accessed by any programs other
than the tar program (and of course /sdb1 is the shell's CWD).
The shell's history file is in my home dir.

After tar:

4. du -sk dirx  /sdc1/dirx
2904536    /sdc1/dirx
2802124    dirx

So, why this size inflation by 104MiB ?

I repeated the process twice. Same difference.

Other dirs tarred in this way from sdb1 to sdc1 do not show this 
discrepancy.

Dirx contains mp3's.