Why does templated interface function return something different than final function?

[Timoses via Digitalmars-d-learn]

Why is the interface templated function not also returning the 
class C toString return value "in C"??

        interface iface
        {
                void toString(scope void delegate(const(char)[]) sink) const;
                final string convert() inout
                {
                        import std.conv;
                        // assert(std.conv.to!string(this) == "in C"); // fails!
                        return std.conv.to!string(this);
                }

                final string convert2() const
                {
                        import std.conv;
                        assert(std.conv.to!string(this) == "in C");
                        return std.conv.to!string(this);
                }
        }

        class C : iface
        {
                void toString(scope void delegate(const(char)[]) sink) const
                {
                        sink("in C");
                }
        }

        void main ()
        {
                iface i = new C();
                import std.stdio;
                writeln(i.convert); // "app.C"
                writeln(i.convert2()); // "in C"
        }

It seems like `inout` triggers some odd behaviour??