> How reliable is this uptime?

IT seems to be quoting incident reports for the duration of each outage, so there is accountability in terms of being able to verify all the details of what they are counting.

> and why it's sooo different from gh's official status numbers?

Maybe this is counting any period with any service showing any level of issue as a complete fail, and the official numbers are cherry-picking a bit (only counting core services? not counting significant performance issues that the other count does because things were working, just v…e…r…y … s…l…o…w…l…y) or averaging values (so 75% services running at a given time looks ¼ as bad in their figures), the two sets of calculations could be done with a different granularity, …

In other words: lies, damned lies, and statistics!

The only way to know is to know how both are calculated in detail, and that information might not be readily available.