Person A does not know because they don't whether Person B is ready. If Person A was not ready themselves they would know the answer is No. We assume here that

Person A does not know because they don't whether Person B is ready. If Person A was not ready themselves they would know the answer is No. We assume here that people know wherher they themselves are ready. Person B then can safely assume Person A is ready and if they are ready themselves they are now indeed ready.