对某人很失望是be disappointed in you还是with you?in和with有什么区别?

对某人很失望是be disappointed in you;
对某事很失望是be disappointed with sth.那为什么有很多地方说be disappointed in / with sb是对某人感到失望呢？能解答一下吗？严格讲还是 in sb with sth.,disappointed with sb 用到的情况比较少