Presumably, those estimates are just for the Space Shuttle Main Engines (SSME/RS-25), give that's what Rocketdyne was manufacturing and propulsion is one of Marshall Spaceflight Center's fortes.
If so, the risk for the entire system (shuttle) should be higher, not lower.
Edit: To be clear, I wasn't saying it was lower.
Sorry, I didn't mean to refer to you; I should have been more clear. I was referring to NASA's or anyone's estimates for the shuttle.