JSON parsers all agree what JSON actually is (except for the brown colored fields in the compat matrix, which are actual JSON compat bugs).

JSON parsers may also permit various additional variations which are very explicitly not JSON. This means that they may accept a handcrafted, technically invalid JSON document. However, a JSON encoder may never generate a document containing such "extensions", as this would not be JSON.

This concept of having parsers accept more than necessary follows best practices of robustness. As the robustness principle goes: "Be conservative in what you do, be liberal in what you accept from others".