Ubuntu LTS repository package update policy

I recently realized that even though Ubuntu LTS is receiving 5 years of support, a lot of the packages in Ubuntu LTS package repository are not updated with the latest stable releases.

For example, php5 package in Lucid (10.04 LTS) is at 5.3.2-1ubuntu4.17; while the same php5 package in Precise (12.04 LTS) is at 5.3.10-1ubuntu3.2.

Why the discrepancies? Update one while not updating the other despite an Ubuntu release is still being supported?

And, what are the package update policies as to whether to update a package or not.

The update policy is for security updates and critical bugfixes only. New upstream releases are not part of that. This isn't an LTS-only policy; any sane distro that aims to be a stable basis for infrastructure will have a very similar policy.

I've researched and found acceptable answers to my question:

Ubuntu team outlines the decisions on when or if a package should be updated at their wiki on stable release updates.

Also, the procedures in determining if a package is worthy of a security update are outlined at the security team update procedure.

See also: Related discussion on AskUbuntu