These numbers are useful proxies for how likely you are to have your work disrupted outside of your own control.

If you do something 100 times a day against a four-nines service, you can reasonably expect that everything will succeed.

If you do something 10,000 times a day against a two-nines service, you can expect to hit a substantial number of errors during that day, or even have long periods where your work cannot happen at all.

People aren't frustrated with Github because Github has 98% uptime or whatever the specific number is. They're frustrated because it regularly interferes with their ability to work. The 98% number is just a concise way to say it.

But is often wrong itself.

Before 9's became a thing, people built systems that required outages, and those outages would happen outside of business hours.

However that's also why some transactions had to complete the next business day after they were registered. Because the whole business was running on offline processing (aka batch processing) that could be interrupted for upgrades, but had to be completed by the start of business the following morning.

My dad did one of those jobs, and my brain has made a bigger deal out of the times I awoke in the middle of the night to find him on the phone at the kitchen table at 2 am dealing with a war room call because an upgrade broke things that needed to be done in 5 hours. It probably only happened 3 times that I know about, and I probably knew about at least half of them, but it felt like it happened twice a year.