What's the difference between auto a = A(3) and A a(3)?

Suppose I have:

struct A
{
    A(int x) : m_x(x) { }
    A(A&&) = delete;
    int m_x;
}

and:

A a(3); // Ok
auto a = A(3); // Error: function A(int&&) cannot be referenced - it's a deleted function

Why does the latter call to the move constructor? Why are these 2 statement different in terms of generated code?