They just don't average number, but have a graph. And they have it for 60Hz LCD and 90Hz OLED listed separately. We knew about this feature before it came out officially (could opt in for data collection). And we feared the same thing you say, but Valve did actually solve it by providing a graph. A game that is mostly stable will sit in one number mostly. Games that have a bit problematic FPS will be lower and all over the place. It's still a rough estimate, and I don't think a more detailed view is not needed nor possible, because people play with different configuration. This is not meant to be a perfect number, its just to give an idea how it performs.
It's just a bit hidden, that's the problem:
