copying files with cp

Copying a file of about 700kb inside a "-o loop device" of a ext2
filesystem
i have had a problem.
One byte changes (in an arbitrary position) from 0xe0 to 0xa0.
Very strange.
I verify it with Hexdiff.
I execute again the same command and it doesn't happen.

kernelversion 2.4.18
redhat 7.3

cp (fileutils) 4.1
Written by Torbjorn Granlund, David MacKenzie, and Jim Meyering.
Copyright (C) 2001 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

WHY?

THANK YOU