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That's not what that column is supposed to mean afaict. The way I read it is it's showing that if the website requires hundreds of different parallel backend service calls to serve the page load, what's the probability a page load hits the p99 instrumentation latency?

We have a similar chart at my job to illustrate the point that high p99 latency on a backend service doesn't mean only 1% of end-user page loads are affected.

Ah, I see. So, for example, if one page request would result in 190 different backend requests to fulfill, then the possibility that at least one of those subrequests exceeds the 99th percentile would be 85.2%. That makes a lot more sense.

I recommend watching Gil Tene’s talk, I think he explains the math better than I do: https://www.youtube.com/watch?v=lJ8ydIuPFeU

But what if the 100 page loads are just a sample of the population?