Why individual package repositories?

[Ylan Segal]

I noticed in the kubuntu website that amarok 1.4.1 and kde 3.5.4 have
been released for kubuntu. Great! I installed them right away.
Everything worked great, however I do have the following question on
repository policy:

Why does each of this updates have it's own repository?

My understanding is that after a (k)ubuntu release, the packages pretty
much stay as they are (for main, universe and multiverse repositories),
except if there is a security problem, in which case the patched
packages are set up in the security repository. This is done with the
intent to promote stability.

Now, since many people absolutely can't wait to have the new packages,
the backports repositories exists to include newer packages that will be
included in the next (k)ubuntu release, resulting in the newest bells
and whistles with a possible less stable system.

Assuming the above is correct, then why were the new kde version and
amarok released on their own repositories instead of backports?

I think I am missing part of the picture, being new to (k)ubuntu and all.

Thanks,

-- 
Ylan