apt-get update updates your whole system (if that software is part of any repository that is). Not just zend server.
From the zend server website:
Upgrading Zend Server
Upgrading Zend Server can be performed using the following programs:
To perform this action you must have root privileges.
It means that the update of this bit of software is done by
update manager. If there is no update
apt-get will not show any results.
Package name seems to be
x.y is the PHP version. You can check your repository since there should be a line included for this installation:
deb http://repos.zend.com/zend-server/deb server non-free
All the packages inside this repository belong to zend server and
apt-get update will check these for newer versions.