Ok, there is a 1GB swap partition, but that actually increases the discrepancy rather than shrinks it. That means that potentially the guest OS is using 20GB rather than 19GB and vCenter is still reporting 11GB.
df -h won't show swap as its not a mounted filesystem. Looking at /etc/fstab file will show if some partition should be being used for swap and the 'free' command will show the aggregate size of any swap partitions that have been activated.