Well, that’s not a normal update, that’s a complete distribution upgrade. It shouldn’t actually be possible to do that without changing the repositories being used in the sources.list file.
Unfortunately, there isn’t any planned support for Ubuntu 13.04, as it’s not an LTS release. Ubuntu 12.04 is the newest supported Ubuntu version until 14.04 comes out (which will also be an LTS release).
Sorry, I know that’s not the news you wanted to hear
I’m not actually sure how that might have occurred though… just running an “apt-get upgrade” just pulls in packages from the repositories defined in the /etc/apt/sources.list.
If you want, feel free to paste in the contents of that file, maybe that’ll make it clearer as to how that occurred.