The speed the NHC publishes is the estimated speed of motion at the time of the advisory ( correct me if I'm wrong anyone ) but they may also take into account the average speed between the last advisory and the current one. The speeds that I am quoting are those average speeds, between the last advisory position and the current advisory position i.e. say the distance from point A to point B is 68 miles. It takes the storm 6 hours to move between these two points, hence the average speed was approx. 11mph. Taking into account the NHC's level of uncertainty of any advisory position that they publish, the actual speed may be more or less than the original speed quoted. This may result in them making a conservative estimate of speed at the current position, in this case 8mph.