That is true in theory. In practice, when using the motor as a generator, the operating conditions are different, and so are the efficiencies. The load side is

That is true in theory. In practice, when using the motor as a generator, the operating conditions are different, and so are the efficiencies. The load side is also different, the motor controller will not act the same way as the inertia of the car. For example, even the simple inductance and resistance of the motor means that at the same speed and at the same load angle you will see a lower voltage at the poles of the motor when acting as a generator. When your motor has high efficiency, these differences can lead to a large absolute difference when running as a generator.