I know the composite scores are rounded. The stats for composite would be a little different than cut off totals. If I understand it right the composite it the total of the 4 scores. E+R+M+S=C. So I 91 would be a 22.75, but is rounded to 23 obviously. The cut off does not involve rounding up or down. I'm sure that can effect it at this level.
Are you sure your looking at the 2011 Stats?
4th grade 23 = 100%
5th grade 23 = 99.2%
6th grade 23 = 98.2%
I do think it kind of stinks that they list 2%, but it doesn't appear they have used the 2% number in recents years. You can log into the NUMATS toolbox and get your percentiles for all of his numbers.
Your DS did great!