Hitting the Progressive Grand Jackpot is like filling up a container. Players are continuously filling it up with coins every second. Some with more; some less; others minimal; while a handful max-sing it out. The moment the container couldn't hold on to the weight of those coins, it collapses, and pays out the pre-fixed Jackpot amount.
It's extremely difficult to postulate the same outcome with different amount of bet, different press (fast tab or spin normally or spin after a long while, etc), the timing of the bet, et al.