Github measures/reports the SLA of the individual services.

The external page linked above goes the other extreme and considers it a bad status whenever any individual service is degraded.

In reality the majority of people only use 3 or 4 of the core services the majority of the time but since there's no "core services" SLA/uptime the usability of github for the majority of people is slightly obfuscated.

Part of it is that it considers downtime in any of the services GitHub provides as GitHub being down. So if GitHub had 100 different services, and only one of them was down at any given time (but at least one was always down), then it would show 0% uptime.