It is possible to have scratches that you can't see to well, and you may have missed. You may have tilted you Xbox to where something inside of the Xbox scratches the game, or it may be laser burnt, meaning the laser that reads the game was in one place for a while (i.e. you paused the game and went to go eat, take a shower, whatever, and the laser was continuously reading the one part of the disc, burning it.) which is normally unrepairable.