[ehrman]

If you're talking the realm of five 9s, etc, that's in reference to service availability at some given point in time throughout the course of a year. If you discuss the probability there will be an outage once during a year, the answer is somewhere in the middle, around "more likely than not".

You could use this same probability around a pacemaker. The device is virtually certain (99%) to function at a given point throughout the year, but the probability that the device will not fail over the course of the year is not 99%. If the pacemaker had a 99% chance of not failing once during the course of a year, it would be virtually certain it would not fail during that year.