An Ubuntu release goes through several stages before it actually makes it to the public as a finished product:

• Some time before Ubuntu launches a release it freezes its packages at a certain point.

  
  



• Before a release is out but after the package freezing, work is done mostly to fix all the bugs and issues that there might be in those packages. New package versions are not imported into the repositories anymore after package or feature freezing.

  
  



• Once the release happens additional changes to those packages only happen for bug fixing and security issues. There are no more upgrades done to the packages in the official repository even if new versions of the packages are released.

  
  



• Some packages have an exception to this, such as web browsers (which need to be kept up to date all the time) or certain cases.

  
  

New version of packages are consistently being imported (from Debian) for the next release of Ubuntu, until the next freeze happens and the same process repeats itself.

As an example, you can have a look at the release schedule of 12.04.

You can see that even though 12.04 was released in April, in January 12 something called _Debian Import Freeze_ happened.

This is just the first of many freeze stages happening before the actual release and means that at that point the import of packages from Debian testing or unstable stops and work starts on them to customize and fix issues with them.

No upgrades are done after that point in a lot of packages and the version that package had at that point is the version present and maintained through a release's lifetime.

So even though there are higher versions of the same package in developers' PPAs or in Ubuntu+1 repositories those will only be included in the next release of Ubuntu.

This is done for stability, security and functionality. New bleeding packages being imported all the time to the main repository would mean issues and a lot more problems to be solved. A freeze in the packages version helps to sort that out and make Ubuntu safer and more stable for the end user.

A new version of Ubuntu is released every 6 months, so every 6 months new packages are prepared, tested, customized and released with a new version. Future versions of a packages can be installed in your system via a PPA or just by downloading it from a web site, but the version of the package in the official repository remains the same.

For more understanding and an interesting overview of what happened to Ubuntu from 10.04 until the launch of 12.04 have a look at ReleaseSchedule - LTS to LTS and Stable Release Updates page for a complete overview and explanation of an Ubuntu stable release.