When a version of Ubuntu is released and is considered stable the stable release updates just don't get uploaded into the archive. So let's say a piece of software needs an update, after they go through the stable release update process they get uploaded to the "proposed" pocket. (Step 4 in the linked document):

Upload the fixed package to release-proposed with the patch in the bug report, a detailed and user-readable changelog, and no other unrelated changes.

Here's a list of what some of the updates look like. Then the people who have enabled the proposed section (including the person that uploaded it) test it, this usually takes about a week, sometimes less.

After that the update is pushed via the normal update channel and you get it via the update manager. Basically it's just a place to test updates before they hit the general public.

More info:

https://wiki.ubuntu.com/StableReleaseUpdates