FWIW, DD9 thought the ITBS Reading was very easy compared with the NWEA Reading. One key difference is that the NWEA test is adaptive, and if the child is doing well, the questions can get more difficult, possibly giving a child a RIT score well above the minimum number needed for a 99 percentile. She scored a 99 percentile on both, but as Zen pointed out - 99% is pretty vague, because it encompasses everything above a given number. HTH!