We have a total of 119 IP addresses hosting websites right now. Of this, 4 had less than 99% uptime and 8 had between 99% and 99.9% uptime in the last 30 days. That means there are 107 IP addresses which had an uptime of over 99.9%.
So the 99.9% uptime is true for most websites most of the time. Ideally, we’d provide those kinds of figures to all sites all the time, but between frequent DDoS attacks and the occasional system error this can just happen.
My uptime checks poll the IP address to see whether it responds to connections. Most outages either come with a Connection Refused or Connection Timeout type error on the entire IP address, which is picked up easily by my monitoring.
Other issues, like home directories not being available or the database server being unresponsive aren’t so easy to monitor.