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...
Not even computers are perfect. They fixed the rollbacks with Ka and by developing the overlaunch system. I can't say for certain that it never rolls back, because I don't know, but I know the system was implented to eliminate rollbacks, or at least eliminate the high frequency of rollbacks.
As already stated, it's all variance. Temperature, wind speeds, passenger weight, mechanics, etc all play rolls, and sometimes the physics just works out where there just isn't enough momentum to clear the top.
The kinetic energy (KE) of the moving ride has to be converted into gravitational potential energy (GPE) at the top of the drop, and back again to kinetic energy as you go down. For the train to reach the maximum height, it therefore needs the right amount of KE generated from the launch.
KE = 0.5*m*v^2 where m = mass and v = velocity
So the main two variables are the mass of the train and the speed at which the train is travelling.
But it's more complicated than this since the velocity is affected by friction from the track, drag from any wind etc. etc.
i know that dragster rolls back more than kingda ka. dragsters also gotten stuck twice at the top of the hill and with kingda ka there magnets on the top to make sure its at least pushed one way or the other.
Technically, brakes would only make the chance of getting stuck more probable, although the positioning of the brakes makes that kinda impossible.
But, something that has been mentioned.. somewhere on the boards.. the brakes on KK are only meant to slow down the train because of the launch speed. They intentionally launch the train faster to prevent rollbacks/stalls. Hence.. the brakes.
And I'm not sure about all parks but in most parks, even though a rollback may not be the intention of the attraction, the intention is usually at least to create the sensation that the train just might not make it over so they usually want to make it a close call (or at least feel like it is). It's the fear factor behind it. Thus, this allows for the occasional (or more than occasional rollback)
I have never experienced a rollback, but I did make an epic call at Cedar Point. I pointed at the train and said that one is going to roll back and sure enough it did. A few people in line looked at me funny like I was somehow behind it.