I would think that it had to do with the weight of the train for each launch. The train is weighed and the drive system then determines how much power it needs to launch the train.
Its somewhat hard to believe that they can put a time on 0-128. If they were to get the train to launch 0-128, in 4sec, everytime, that would be a tremendous amount of stress on the drive system!
Obviously the heavier the train, the more power and longer distance it needs to get the speed, but i assume that they have a maximum on how much power to let the system have. So therefore they just make the launch track long enough to get up to speed, even at the MLW(Maximum Launch Weight).
Thats only my guess, but i believe it makes sense.