Any risk estimate makes implicit assumptions, so they are conditional probabilities rather than absolute probabilities. It is assumed that Amazon engineers mean

Any risk estimate makes implicit assumptions, so they are conditional probabilities rather than absolute probabilities. It is assumed that Amazon engineers mean to prefix these data loss numbers with "under normal operation of our services". Definitely, exceptional and unpredictable incidents make the absolute risk higher. But in the absence of historical data about the rate of occurrence of such events, it is impossible to make a reliable assessment of absolute risk. Not only is it impossible to assign a realistic probability to events that you know can happen (eg. catastrophic bug), but many of the possible incidents are events you are not even aware can happen (eg. are you going to factor the risk of a meteoric crash destroying your data centers?).