C++11 move semantics and rvalue reference

Consider the following code:

class StringTokanizer 
{


private:
    char m_delimiter;
    std::istringstream m_string;


public:
    explicit StringTokanizer(const std::string& str, char delimiter) : m_string(str), m_delimiter(delimiter) { }

    template <class Container>
    operator Container ()
    {
        Container container;
        for (std::string token; std::getline(m_string, token, m_delimiter); )
        {
            container.insert(container.end(), token);
        }
        return container;
    }
};

This is the usage:

vector<string> tmp = StringTokanizer("123 456", ' ');

When debugging the following happens:

At the return statement of conversion operator

1. new vector constructed from container by moving
2. container gets destructed

After function return:

1. tmp is constructed by copy constructor

My question is why isn't tmp constructed by move constructor ?

As I understand things function return type is rvalue and should be moved.