You reasoning is somewhat flawed. 2% of the children who took the test missed one question or fewer in reading. What if most of them actually got perfect scores in this subsection (90 steps, in your analogy)? Then they would have two questions to miss in math or some section where your child scored perfectly (they would only need 48 steps to get to 138), and although they would be "behind" in those areas, they would still be "even" in the total battery. Likewise, if the one percent who did as well or better in math got perfect scores, they would have two points to miss in reading or some other subsection to stay "even" for the total. Right there we have the possibility of three percent, not one percent, of the other children having the same total battery. I don't know how they deal with rounding when reporting their percentile ranks, but that could easily account for another percentile or two. I'm having a bit of an issue trying to stretch it all the way to seven...but there is certainly no reason to think that the total battery should be 99th percentile.
