Comment by brazzy

Determinism is relative. "Non-deterministic" means that an observer, to the best of their ability, cannot predict the result even with full knowledge of all inputs.

This can be used as an abstraction for a theoretically deterministic system that is too complex to handle.

But even if you take "ability" and "knowledge" (of all inputs and the inner workings of the system) serious, there is always the possibility that there are factors you simply don't know. Maybe your model is incomplete. Maybe there are as-yet-unknown physics that make even radioactive decay predictable. So really, it's always an abstraction.