apt-get does not work the first time but works the second time

i install ntp like:

apt-get -q -y install ntp=1:4.2.6.p3+dfsg-1ubuntu3.1

It failed saying:
WARNING: The following packages cannot be authenticated! libcap2 libopts25 ntp E: There are problems and -y was used without --force-yes

Afterwards i ran,

apt-key update

and ran the same commad with --force-yes:

apt-get -q -y --force-yes install ntp=1:4.2.6.p3+dfsg-1ubuntu3.1

Thereafter running apt-get purge and reinstalling ntp runs. "without" --force-yes

apt-get purge libcap2 libopts25 ntp
apt-get -q -y install ntp=1:4.2.6.p3+dfsg-1ubuntu3.1

Also i created a fresh VM and could not reproduce the issue. On a fresh VM, the same apt-get command runs the first time, without "--force-yes"

Two questions, why does running apt-get work the second time and cannot reproduce the error ?

full errors and sequential steps at, https://gist.github.com/3017966