Reality is more complex than any model.
That's why we build models. They are, by their very nature, simpler than reality.
Everything is in flux, and you can choose to model that or not model it.
At the first level, you just model the current state of the system.
Then you begin to model the events that change the current state.
Then you begin to model changes in the shape of the problem domain.
Models all the way down ...