At certain points you can hear the music fluctuating pretty rapidly, but it's the same stuff. The piano note at the end cuts through especially well only because it sounds like the music was fed through a mono speaker or an equalizer (kinda the same reason why songs sound different on the radio than they do on your iPod; on the radio the parts are balanced out more, making it sound more "flat")
It's even more discernible when you listen to the HD version of the game, as that music is even more fine tuned than the online version of the real Sub2 (MS's HD ambients are better naturally though; hence "HD"
)
