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I conducted a small test on the initial stages of a siege, using a recent online tournament (OT) as an example. Out of 30 attempts, the wall was breached approximately an equal number of times at points 2 and 4 where units were stacked. The wall was never completely breached in any of the attempts. In one case, lightning spells had no effect on the defending dwarves. This suggests that each event is considered separately and occurs with a set of probabilities. Therefore, it would be very difficult for the tournament organizer to reproduce a complex battle where luck played a role 2-3 times in an official tournament, even after a very large number of attempts.
With best regards, Ergon.