Comment 4 for bug 494968

postinst has:

adduser --system --quiet --group --no-create-home ntp || :
chown -R ntp:ntp /var/lib/ntp /var/log/ntpstats

If adduser fails for any reason (and with "--quiet" and "|| :" it's hard to know), then chown would fail.
Can you (or any of the 9 people affected by this bug) reproduce after purging and reinstalling the package ?

sudo apt-get purge ntp
sudo apt-get install ntp