I have only seen the ITBS, but from what I've seen, I would guess that what you're seeing with these 1300+ scores is an extrapolation, as I am quite sure I've never seen a reading passage that could possibly be higher than 1200 or so (and I've actually taken the ITBS as an adult up to the 10th grade level). I'm guessing that what you're seeing here is noise on the high end of the scoring.