I suspect a major part of this is the "teaching to the test" that public school curricula does. Granted, I am basing my suspicion just on personal experience of knowing people who were educated in Montessori type schools always saying they did poorly on tests.(Plus as regards myself, I was an excellent test taker who felt the downside repercussions of that in that teachers seemed then to expect more from me than I was capable. So I just don't trust standardized test taking.)

It's mentioned in the article:

Private school proponents have often argued that comparing standardized test scores of public and private schools is unfair because public schools are held more accountable for test scores and tend to spend more time teaching tested subjects while private schools offer enrichment programs like art and music.

As a non parent, I am just not prepared to blame parents who want to support vouchers, just for this reason, my bold:

The report adds to mounting evidence that voucher programs across the country, which are often seen as an alternative to inferior public schools, are producing mixed academic results. Recent examinations of programs in Indiana, Louisiana, Ohio and Wisconsin have drawn similar conclusions.

On a positive note, families who participated in the Washington program reported that their private schools were safer by a large margin.

What's that worth? Whether real or not? I'd fight for that feeling if I were a parent. Just can't blame them. i am reluctant to criticize parents having full choice of what to do about their kids precisely because I chose not to take on that awesome responsibility.