为什么即使正确设置了返回值非null，也要改造返回null'LiveData'的方法？

由于您在存储库类的类级别使用LiveData对象，因此您的方法无需返回任何内容。相反，它的责任是更新该LiveData对象。

public class ProjectRepository {
    private static final String TAG = "ProjectRepository";

    private ProjectRepository instance;
    Context context;

    public ProjectRepository(Context context) {
        this.context = context;
    }

    public MutableLiveData<List<PojoDivision>> data = new MutableLiveData<>();

    public void getDivisionList() {
        ((RetrofitConfiguration) context).getDivisionRestApiWithAuthentication().getDivisionList().enqueue(new Callback<List<PojoDivision>>() {
            @Override
            public void onResponse(Call<List<PojoDivision>> call, Response<List<PojoDivision>> response) {

                if (response.isSuccessful()) {
                    data.postValue(response.body());
                }
            }

            @Override
            public void onFailure(Call<List<PojoDivision>> call, Throwable t) {
                Log.d(TAG, t.getMessage());
            }
        });
    }

} 

此类的客户端需要1）观察LiveData对象，以及2）调用该getDivisionList()方法以触发更新：

class MyFragment extends Fragment {

    private ProjectRepository repository;

    private void setRepository() {
        Context context = getContext();
        if (context == null) return;
        repository = new ProjectRepository(context);
    }

    public void observeData() {
        repository.data.observe(this, new Observer<List<PojoDivision>>() {
            @Override
            public void onChanged(List<PojoDivision> pojoDivisions) {
                // do something with updated data
            }
        });
    }

    public void triggerUpdate() {
        repository.getDivisionList();
    }
}