Average movement is calculated by finding the distance between the current storm position and the storm's position the noted number of hours prior to that. That number is then divided by the number of hours to get the average storm movement per hour. While the storm movement is rounded to the nearest tenth, it does not indicate precision to that decimal place. It's only there to help show when there might be a slight change in speed that might not be reflected if it was rounded to the nearest whole number.About Distance and Bearing
The distance we calculate is the great circle distance. However, for the bearing we use the bearing along the rhumb line between the two points, which is a constant value, rather than the initial bearing along a great circle path between the two points. Along a great circle path the bearing changes as you travel along it, significantly for large distances, which is why we choose to use the constant bearing along a rhumb line which allows us to get a movement calculation that can have a nice cardinal direction.