question: why are two consequent runs of rsync -a --only-write-batch=patch /src/ /dst/ producing two different patch files (when src and dst are read-only)

[SSSSeb]

I am using rsync to generate the 'diff' between to subtrees ( dvd loopback read-only mounts )

mount -o loop dvd1.iso mnt1
mount -o loop dvd2.iso mnt2

'rsync -a --only-write-batch=diff1 ./mnt1/ ./mnt2/'

running a second time the same command (mnt1 and mn2 are read-only and did not change)

'rsync -a --only-write-batch=diff2 ./mnt1 ./mnt2/'

... diffing the two diff1 and diff2 files shows byte 11, line 1 to differ

is this a serial number ? a timestamp ? where in the code is this byte 11 being written to the archive ?