反义疑问句如何辨析

There is little doubt in your mind that he is innocent,is there?这一题的答案解析是：little 为否定词,因此附加反意疑问句应该用肯定形式.

It's impossible that you achieve your goal without effort,isn't it?这一题的答案解析是：impossible为否定词,因此附加反意疑问句应该用否定形式.
我想知道这两句为什么有这样的区别?