Thats the rub.

Once you dig into the details what does it mean to have 5 9s? Some systems have a huge surface area of calls and views. If the main web page is down but the entire backend API still is responding fine is that a 'down'? Well sorta. Or what if one misc API that some users only call during onboarding is down does that count? Well technically yes.

It depends on your users and what path they use and what is the general path.

Then add in response times to those down items. Those are usually made up too.