1% is well within the normal variation in strength that parts exhibit. Even with modern materials and precision machining, parts will always vary in performance

1% is well within the normal variation in strength that parts exhibit. Even with modern materials and precision machining, parts will always vary in performance to a degree that is known. That said, predicting critical points is very deterministic process. Strengthening the critical point that failed in this test by a reasonable margin should, for all reasonable likelihoods, eliminate this point of failure. Predicted performance has error bars, and actual performance has scatter. If we bump up the strength, we can make sure the tail end of that scatter has less overlap with the failure strength scatter (theoretically we can never reduce the overlap to zero). All of experimental mechanical engineering is a comparison of statistical odds. It's quite possible that if they tested another identical fuselage, it would have passed the test and this would have never made the news, rightly.