Redundant namespace qualifier on the call

Question

Why is Categories::ff(Categories::a) redundant?

namespace Outter{
    namespace Categories{
        struct V {};
        V a;
        enum class T { A, B };
        void f(T a);
        void ff(V a);
    }

    void f() {
        auto vec = vector<float>{};
        Categories::f(Categories::T::A); // Nothing redundant.
        Categories::ff(Categories::a);   // Redundant Categories quantifier
                                         // on the call itself.
    }
}