I saw first-hand how it comes to pass, from collaborating with NASA engineers years ago. In order to ensure that a critical mission-goal is met, with, say, 95%

I saw first-hand how it comes to pass, from collaborating with NASA engineers years ago. In order to ensure that a critical mission-goal is met, with, say, 95% confidence, every subcomponent must be much more reliable, as there will be many ways to fail. The net effect is that often the overall perceived reliability turns out to be much better than the requirement. This is especially so for systems that have already achieved some measure of success, as a number of those subcomponents will have already done their jobs completely.