只是因为他们觉得一个外国人说这么地道的（“哪里哪里” is an extremely idiomatic&cultural-specific expression)中国话觉得非常可爱。你这样说一点错都没有。把这种笑当成鼓励吧，虽然他们也许口头上没有表达出来(We Chinese tend not to be vocal with encouragement but believe me their laughing at you says exactly "wow we're impressed")。

because they were so amazed for what you said.........the laugh might = how come your chinese is so good?!so,it's a 'benevolent smile'(善意的微笑).

你的回答没有问题，只是可能他们觉得一个外国人这样回答有点奇怪，或者他们的笑是善意的。

em..the story is:有个外国人参加一对年轻夫妇的婚礼，在婚礼上这位外国人赞美新娘：你真美。新娘谦虚地说：哪里哪里。外国人一听，以为她是在问自己什么地方美，where=哪里，于是回答道：哪里都美。 that's all.

there is a short funny story about a foreigner and the word"哪里哪里",I think they must be think about that.but dont be puzzled,your expression is right.