By “developed countries”, is this the UN’s term, which replaced First and Second World? Remember, they still define Red China (formerly “Second World”) as “developing”.
The UN is not really a serious reference for anything. It’s just a role-playing fantasy game.
The Organisation for Economic Co-operation and Development (OECD) is taken seriously at providing economic definitions and data.
Most economic data is abstracted selectively to serve some political purpose. Probably no one would be much interested in aggregate economic data if there was no political opportunity to be made from it.