I think this is about the granularity of SLAs and monitoring. If a provider promises that, overall, 99.5% of all requests will succeed, but the 0.5% errors are

I think this is about the granularity of SLAs and monitoring. If a provider promises that, overall, 99.5% of all requests will succeed, but the 0.5% errors are all concentrated on some few customers / regions / AZs, customers can have a very bad day. So this is about promising each customer that 99.5% of all their requests will succeed, and monitor in a way that makes sure you can keep that promise.