Anyone can score a website out of ten. The question is what the number rests on, and whether it survives somebody checking it.
The emissions half is the clearest example. It would be easy to publish a single gram figure and a global per-visit average alongside it, and a great many sites do. We do not, because an average tells you about the web rather than about you, and a bare number with no range implies a precision the method does not have. What comes out instead is a calibrated estimate with a 90% confidence range and a stated honesty tier, so you can see how much of it is measurement and how much is model. The method is published.
The performance and experience halves are the same discipline pointed at different evidence: published research on what speed is worth, in the science behind the framework, and a standard with testable criteria rather than an opinion about usability.
We run it against this site before we run it against yours, and publish the result whether or not it flatters us.