The doubt is in whether she was in control of herself when she killed the children. They are saying that even with testimony that she was aware of what she was doing, the jury DOUBTED that. They were not convinced that she was sane (enough), and her defense was a lie.

At this point, she is essentially free, until they decide to prosecute again. If the lone holdout had voted with the jury, at least she would have been locked up in a hospital.

“At the end of the day, there was so much doubt,” the woman said. “The prosecution didn’t have a figurative smoking gun. There was not a single moment throughout the presentation of that case where you could say ‘Yup, 100% I’m certain she did it’ or that she didn’t do it. There was too much gray area.”

"At the end of the day, to not follow that reasonable doubt and to acknowledge that it's there, you morally and legally cannot convict somebody for that. And that was therefore a violation of her constitutional rights," the juror said. "Essentially, that is what we were there as a jury to do is to uphold that Constitution and give someone a fair trial of their peers. And I don't necessarily think she was getting that with that juror. It's not simply because of the fact that he had an opposing view or this person felt like she was guilty. It was the inability to back that up with any logic."