Downloaded -flat.vmdk from snapshoted machine is different everytime

I have a virtual machine running on VMware ESXi 4.1.0, machine has one Thick provisioned disk stored in VM forder (residing on local datastore). I am planning to move the virtual machine to another ESXi host with minimal downtime - I was planning to shutdown vm, make snapshot, start it up again, copy all files modified before creating snapshot to new ESXi server and some nights later when actually moving the server copy only files changed after creating snapshot.

I have created several snapshots of the machine. But always when I download vmname-flat.vmdk this file has a different MD5. I have downloaded it with vSphere Client and with Veeam fastSCP several times with each - no 2 files are the same. Why so? Is this by design?

How it is possible to copy all data till latest snapshot while machine is running and copy only latest delta while machine will be turned off?