thats probably not a good explanation, why it happens im not sure but maybe its the expansion of spacetime or subatomic quantum quackery
It's basically right. For example we know the weight, speed and position of all the celestial bodies in our solar system, along with the laws of physics to predict them. But we still can only predict them a million or so years in the past and future. The problem becomes that if you start your simulation with some values, the Earth falls into the sun. If you position the Earth a nanometer off, it gets flung into space. Another nanometer and it crashes into Saturn. The result we compute is basically "random".