I'm not totally sure of what I'm saying, but if you're on an LTS such as 12.04, you may not get updates to all your software. You'll get updates if they relate to security and possibly for major bug fixes.
Originally Posted by michael-piziak
The same goes within the life-span of non-LTS releases.
New releases will get new software if such software is available in time and passes whatever quality controls the maintainers impose. In other words, just because a software package has been updated by its creator doesn't mean we'll get it ASAP.
Re. gimp, you can run apt-cache policy gimp to see what version of software is available on your particular version of Ubuntu. I'm on 13.10 and I see:
So this answers your question: "is the software center behind, or is the 2.8.4 not available for Ubuntu yet ?"
[08:44 AM] ~ $ apt-cache policy gimp
500 http://archive.ubuntu.com/ubuntu/ saucy-updates/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu/ saucy-security/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu/ saucy/main amd64 Packages
[08:46 AM] ~ $
As aj points out, there's always the ppa route or even installing from source but then it's up to you to deal with any issues that arise.