This has been explained in several posts in the forum. Each variant binary has a creation timestamp for the firmware certificate and for cmt certificate. So some variants have been created later than the other so they have never timstamps and allow flashing 'as system think its a upgrade) (cmt firmware is same in european variants, so it always succeeds, untill someone flashes chinense cmt (which was always created later than european/vanilla cmt, so it never can be downgraded because of timestamp certificate check).