Actually, i think the distinction between video game song and real song is more subtle.
A real song for listening to has a lot it needs to accomplish. It needs to bring the listener in with a nice tune, and more importantly, it must support itself with insturments to give a full texture. A video game song, however, often has fewer insturments, relies more on melodies, and in the cases of modern video game music, often has reverb or some kind of ambience applied to it. Heavy use of reverb (which many of my songs do, because, well, I like reverb <_<) seems to create a sense of actually being inside the music, which in the case of a video game translates to being inside the video game.
Or I could be completely wrong
