The problem? And all too-common problem with anything to do with information and analysis: human error. Despite the high-stakes ("months and years of freedom gained or lost"), researcher Emily Owens found that the system was generating errors in 1 in 10 trials, even though there were multiple opportunities for the data to be reviewed and corrected.
And what did people find so hard about the worksheet? Simply looking at the right number!"
Here's the Slate Article.