Rolling the Percentile Dice

I'm all for taking care before violent criminals are released from incarceration, and that concern is heightened where they have killed and may kill again, but what should I make of this:

Why was Hanson considered for release when he had apparently been evaluated in prison as being an inmate with a 91% chance of reoffending and an increasing tendency towards violence?

By what scientific method, dare I ask, can one establish such a specific probability figure?