100% of the time when I run a test with WPT (even at the fastest speeds) it is slower than the actual load times I see. Sometimes its off by maybe 20% but sometimes it can be 300% or more. I understand it’s a synthetic test and it can’t be perfect but I always have wondered this and never asked.
Also, when trying to improve CWV for SEO purposes should I be testing using one of the mobile phone emulation options (and what bandwidth) since Google does mobile-first indexing? When doing that, it’s rare for me to get all three metrics in the green zones.