They can only calculate "so much" since they really only have a rough weight figure of the train, temperature, wind, then you have all the other variables, sudden change in weather (cold/rain can increase friction by cooling the bearings), air resistance (people put hands in the air), the hydraulics aren't 100% equal every time, viscosity in the oil, etc.
There are so many slight variables that the computer can't monitor that can change the speed on the launch and make the ride roll-back. Also I think that the computer look at the last few launches and make the calculations based on these, then if there is too many changes it might not compensate enough...