data loss in conversion to or from vpc

Given the script script, I would expect that out.raw* files would
be bit for bit identical. And indeed, a little test image I made, they were.

However, when I tried it on a oneiric compressed qcow disk, they were not.
With 'in' of oneiric-server-cloudimg-amd64-disk1.img, the
output looks like:

f4a076d62bf13a5ba0c9c82496fe3303 /tmp/convtest.vpc
c1f6e09e96361c4f56aea0b9ea845a36 /tmp/convtest.raw
c1f6e09e96361c4f56aea0b9ea845a36 /tmp/convtest.raw.from-file
0f4b00a770aa795b71de4ca3311898fa /tmp/convtest.raw.from-vpc
-rw-r--r-- 1 2147483648 2011-10-26 14:31 /tmp/convtest.raw
-rw-r--r-- 1 2147483648 2011-10-26 14:31 /tmp/convtest.raw.from-file
-rw-r--r-- 1 2147991552 2011-10-26 14:31 /tmp/convtest.raw.from-vpc
-rw-r--r-- 1 709017088 2011-10-26 14:30 /tmp/convtest.vpc

Note, that the from-vpc file is ~ 500k larger than the raw files
and obvisously the md5sum differs as a result.

--
#!/bin/sh
in=${1}; out=${2}
qemu-img convert -O vpc /dev/stdin $out.vpc < "$in"
qemu-img convert -O raw /dev/stdin $out.raw < "$in"
qemu-img convert -O raw "$in" "$out.raw.from-file"
qemu-img convert -f vpc -O raw $out.vpc $out.raw.from-vpc
md5sum $out.vpc $out.raw $out.raw.from-file $out.raw.from-vpc
ls -l $out.vpc $out.raw $out.raw.from-file $out.raw.from-vpc

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: qemu-kvm 0.14.1+noroms-0ubuntu6
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic 3.0.4
Uname: Linux 3.0.0-12-generic x86_64
ApportVersion: 1.23-0ubuntu3
Architecture: amd64
Date: Wed Oct 26 13:53:33 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
KvmCmdLine: Error: command ['ps', '-C', 'kvm', '-F'] failed with exit code 1: UID PID PPID C SZ RSS PSR STIME TTY TIME CMD
MachineType: LENOVO 7417CTO
PccardctlIdent:
 Socket 0:
   no product info available
PccardctlStatus:
 Socket 0:
   no card
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.0.0-12-generic root=UUID=f9832678-e9fb-41c5-8edb-5edd5200ed0a ro quiet splash vt.handoff=7
SourcePackage: qemu-kvm
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/06/2010
dmi.bios.vendor: LENOVO
dmi.bios.version: 7UET91WW (3.21 )
dmi.board.name: 7417CTO
dmi.board.vendor: LENOVO
dmi.board.version: Not Available
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvr7UET91WW(3.21):bd12/06/2010:svnLENOVO:pn7417CTO:pvrThinkPadT400:rvnLENOVO:rn7417CTO:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 7417CTO
dmi.product.version: ThinkPad T400
dmi.sys.vendor: LENOVO