Warum erlaubt TypeScript res.data im success-Block und res.message im error-Block?

type ApiResponse<T> =
  | { status: "success"; data: T; timestamp: number }
  | { status: "error"; message: string; code: number }
  | { status: "loading" };

function handleResponse(res: ApiResponse<string[]>) {
  if (res.status === "success") {
    console.log(res.data.join(", "));
  } else if (res.status === "error") {
    console.log(res.message);
  }
}