Little more info here:
http://www.xbox-scene.com/xbox1data/sep/EEAVyEFuFyZuyXdjmt.php
It appears that modified fw is NOT being detected, and is instead once again due to ‘bad’ discs. Keep in mind that the only full proof way to make a 1:1 backup is to do it yourself. iXtreme is as fullproof as it can be, in that it validates everything it possibly can from a firmware point of view, but a game can have slight data change somewhere, most commonly in DMI. Or it can be due to early copies being watermarked so to speak.