Ubuntu is a derivative of Debian, that means the main core files are the same.
Linux normally comes in Five flavors:
Debian (widely used includes Ubuntu, Linux Mint, and a few others)
RPM (wideley used mainly noted as Fedora Core or RedHat, SUSE just to name a few)
Gentoo (Supposed to be updated frequently and suppose to be highly optimized for use, If you are familiar with ChromiumOS it is Gentoo based)
Slackware (Slax Based, not cutting edge but stable and highly customizable)
Arch Linux (High points: elegance, minimalism, and simplicity)
These are the linux BASE operating systems. With each base comes many different distributions.
Lets take Debian for example, has ~ 50 different distributions, ubuntu being one of them.
EVERY distribution based off Debain use the SAME core files, and NORMALLY the same package handler.
Every flavor of LINUX has the OPTION of running a GUI… but is not required to. there are many different “desktop types” that linux uses.
Ubuntu breaks the use of the desktop environment down into further distributions such as:
Ubuntu (use to be Gnome until 12.00 they switched to Unity which many users coming from Windows find easy to use.)
Kubuntu (Flashy eye candy and uses KDE environment)
Xubuntu ( is intended for use on less-powerful computers, uses XFCE)
Lubuntu (lighter, less resource hungry and more energy-efficient, uses LXDE)
Edubuntu ( eductation based I think it uses gnome)
Mythbuntu (based off of MythTV mainly for media centers)
and the list goes on
As you can see for each Derivative there are Distributions.
You are using Ubuntu, a distribution of Debian, any Debian base files are ALREADY installed on your system.
That being said… Every version of linux has a login page that greets the users for ease of log in, be it a text only log in or a graphical log in. The splash page, also called the greeter, is able to be customized, but in doing so none of the core OS files change, only the LOOK of the log in page, and it can’t actually tell which OS you have installed, it just passes the information to the OS.
With the LTS versions of the Linux OS there is no desktop environment included, you have to install your own, including the graphical login page.
It sounds to me that during the installation or set up of some files (if you were using a guide it probably told you to run commands and that is probably how the graphical interface got installed) that a graphical interface was installed.
I can’t tell you why or how it got installed, as Tom expressed, this Forum is based on helping users with FOG, and the issue you are describing is NOT something that FOG has done. I have installed FOG many times on Ubuntu LTS (the one without a graphical interface) and it has NEVER installed the desktop environment or the graphical log in on its own, only if I issue commands specific to adding an environment or login.
NOTE When you install a graphical log in page you can install one specific to Ubuntu, or any other flavor of linux, it is probable that when the command was typed it had an appendage that told it to install the “debian” back ground on the graphical log in, this can be changed when logged in.
I hope this helps any confusion, if you would like some assistance in understanding how the error occurred or where the graphical interface came from, you really need to visit the Ubuntu forums, I am an Ubuntu nut, but I don’t know everything sadly 
If you would like to revert back to a text only login environment you will need to visit a page such as this:
[url]http://ubuntuforums.org/showthread.php?t=1293350[/url]