Backup stays not the same

Hi,

If I backup an offline VM (no compression) twice, the backup files are not identical. For a 1.8 GB VM (Jessie) the difference is only 313 KB, but why is there a difference at all?

$ ls -l
-rw-r--r-- 1 4294967294 wheel 1557838848 Sep 2 18:06 vzdump-qemu-3019-2015_09_02-18_04_41.vma
-rw-r--r-- 1 4294967294 wheel 1557838848 Sep 2 20:10 vzdump-qemu-3019-2015_09_02-20_09_12.vma

$ time cmp -l vzdump-qemu-3019-2015_09_02-18_04_41.vma vzdump-qemu-3019-2015_09_02-20_09_12.vma > cmp_3019_18_04_41-20_09_12

$ ls -l cmp_3019_18_04_41-20_09_12
-rw-r--r-- 1 root wheel 331529 Sep 2 21:44 cmp_3019_18_04_41-20_09_12


Any clues?

Best,
Andreas