How A Random System Can Actually Be Predictable

The balls in a Galton board fall down randomly. Any time they come to a junction they can take any number of paths and it's almost impossible to accurately predict every ball's trajectory.

However, the system as a whole is predictable.

How can this be? How can random systems actually be predictable?
...
https://www.youtube.com/watch?v=AUmHqD0lGHo