I downloaded a couple of samples from this soundtrack and noticed it differs from the music in the actual game. Here you can hear some of the tracks with an echo in the background, as well as some different "things" (sorry I'm no music wiz so I don't know how else to call it) not present in the game. I believe Square should have made the OST from the game itself, not whatever thing they did in the CD.
Did anyone else noticed this?

Yeah, I noticed it too.

Truth be told, they are the same. Thing is, the PS2 is somewhat limited in power, so they compressed the graphics and music/sound immensely (made obvious by their tinny voices). What you're hearing in-game is essentially the same music, just much lower quality with some of the more exuberant parts of the music pieces cut out.

So Square didn't released an OST with the same quality as the game?

Prior to the release some people ripped the music straight from the game into .psf files. That music should be the same as used in the game.

You can find the .psf files here:
http://tzone.org/~llin/psf/

Search for Final Fantasy XII. Right click on RAR and 'save as'.

To play these files, google and download Winamp.

Then download this file:
http://www.neillcorlett.com/downloads/HE209.zip and install.

EDIT:

By the way, I would say the songs on the OST are actually the way they were supposed to be in the first place. Technical limitations forced them to downsample the music and voice acting in the game quite a bit, like Puyo said. OST sound quality > game sound quality.

If it's quality you're looking for you'll find all of the music to Final Fantasy XII, plus more, in FLAC [lossless] CD quality here:

(scroll down to find it.)
[FLAC Lossless] Final Fantasy [ALL] [MegaUpload] (Thread 39118)

Well at least the music of the Espers battle sounds identical, it's one of my fave. Thanks for those links guys.