The words "safe" and "shuttle" have never really belonged together
If I recall correctly, prior to the first shuttle loss someone had estimated a 1 in 25 chance of catastrophic failure, and it sadly turned out to be all too accurate.
I'd personally guess that this new estimate, coming as it does from NASA, is wildly over-optimistic. (Nothing personal, NASA, I just don't think you're honest.)
Perhaps if they lose another crew they'll finally decide to give it up. And they'll still have one left for the Smithsonian.