Trip durations different for best_guess and maps.google.com?

Thanks for the nice work.

I was using this Python API to compute the trip duration from A to B during the rush hour. What this API reports as a 36min trip shows up on maps.google.com as a 65min trip. Does the `best_guess` traffic model not use real-time traffic data? 65min sounds more accurate given my experiences in this case. Why is the `best_guess` traffic model underestimating the trip duration?