Unique Pointer returns different instances

I have created a Car class.

class Car
{
public:
   std::string NumberPlate;
};

RentCar supposed to keep one instance of Car.

class RentCar
{
public:
   Car& GetCar()
   {
      if (!m_Car) {
         m_Car = std::make_unique<Car>();
      }
      return *m_Car;
   }
private:
   std::unique_ptr<Car> m_Car;
};

In main, while getting the instance of Car, each time, new instances have been given.

int main()
{
   RentCar rentCar;
   auto car1 = rentCar.GetCar();
   car1.NumberPlate = "Temp";
   std::cout << "Case 1 : " << rentCar.GetCar().NumberPlate.c_str() << "\n";
   rentCar.GetCar().NumberPlate = "Temp";
   std::cout << "Case 2 : " << rentCar.GetCar().NumberPlate.c_str() << "\n";
}

Output is

Case 1 :
Case 2 : Temp

Why the function provides different instances?

Also what's wrong in RentCar::GetCar function?