I'm a little confused on how they tested loading speed. I.e. they use webpagetest suite which is made for testing website speed NOT web browser speed. In other

I'm a little confused on how they tested loading speed. I.e. they use webpagetest suite which is made for testing website speed NOT web browser speed. In other words it's being tested on live websites, subject to network latencies and what not? Doesn't seem like a very robust methodology. Am I just confused here?