Here is a link to some information on ITBS scoring: http://www.riversidepublishing.com/scoring/iowa/interpretation.html

Here is what it says about the comparison group:

"Most of the scores on ITBS and ITED score reports are based on norm-referencing, i.e., comparing with a norm group. In the case of percentile ranks, stanines, and normal curve equivalents, the comparison is with a single group of students in a certain grade who tested at a certain time of year."

So what the psych said about comparison to age group vs grade level is completely wrong. I'm guessing that the psych is trying to conform to the guidelines in the Iowa Acceleration Scale Manual but is bungling it.