But it's not an upward deviation from the "happiness" norm, it's deviation from the joy baseline. They're entirely separate systems.

To give an extreme example, someone who's addicted to drugs might receive extreme joy from their next hit, while at the same time being deeply unhappy, and unable to take joy from anything else. That's the hedonic treadmill at the extreme.

At the same time, someone who's very happy might be able to take joy from something simple - like a fried egg yolk with the perfect consistency - because their mind is not preoccupied with seeking bigger and better joy, they may be more free to find it in simple places where it arises naturally.