std::字符串长度()和size()成员函数

I was reading the answers for this question and found that there is actually method called length() for strings (I always used size()). Is there any specific reason for having this method in string class ? I read both MSDN and CppRefernce and they seem to indicate that there is no difference between size() and length(). If that is so, isn't it making more confusing for the user of the class? and