Because racecar? To disprove the myth that stickers add horsepower? (even though the stripes are actually painted I think?) There is an actual reason and its to do with the the no stripe being glitched in GT4 and the GT5 versions is a straight port from GT4 no adjustments to it so it's "raw physics or mechanics" on how it works just makes it faster. Dont qoute me on that though but it's something similar to that and as I remember someone explaining it in a thread somewhere on the forums quite a while ago.