For the purposes of claiming service credits from Amazon, the percentage given on your page is not necessarily useful. Amazon's uptime guarantee applies to the

For the purposes of claiming service credits from Amazon, the percentage given on your page is not necessarily useful. Amazon's uptime guarantee applies to the trailing 365 days. You can claim those service credits at any time. Thus, if Amazon had an incident 2 months ago and have another incident now, then your own window is the range [now - 1.year, now] and you figure that this calculates to a 99.8% downtime for you and claim your credit. Your percentage then necessarily goes to zero because your window restarts. But for everyone else who doesn't claim a credit, the window continues to roll forward.