That does make quite a bit of sense...I had looked at some of the studies earlier and found that on the whole, although the most exclusive schools in D.C. claimed to be participating in the program, most of them didn't have any voucher students, because they said the voucher students couldn't meet their admission standards.
It would be interesting to see which schools had the voucher students and then to look at the scores of all the students in those schools. Of course, other factors SHOULD be looked at as well, such as parent involvement, overall motivation of the voucher students, behavior, etc.