When an Ubuntu version is released, it features get frozen. The updates provided for it is mostly security updates and patches. But major changes of software will not be released (included in the repository) for this release.
That is why the default enabled Ubuntu repositories include security-updates, updates and proposed repositories. The first two repositories include bug-fixes and security updates and proposed is used for testing the newer updates.
Mainly the backports repository's function as said in the Wiki is --
Backports offers a way to selectively provide newer versions of software for older Ubuntu releases. Most commonly, the Backports team will provide new versions of standalone applications which can be safely updated without impacting the rest of the system.
That means, while it was perfectly valid for Canonical for not providing any newer version of standalone software (major releases), You still have a chance to install softwares which are backported. But these are only provided by backporters and they try their best to ensure that, nothing will break. (but it can break the existing softwares sometimes). That means this is a best-effort support and that is why it is labeled as unofficial updates in software-source (
In short words, Backports are not provided by Canonical and you should be glad that some developers tried their best and invested time and energy to bring you new versions of the softwares even after the version is freezed.
I think, I have made it somewhat clear.
Also take a look at this thread