Comment by k__

It is deterministic, but if it's absurdly hard to prove, you just say it isn't deterministic.

Imagine a program that uses JSON and depends on the order of keys in an object. If you input the same object, but with different key order, you'll get different results.

The program is deterministic, but it looks non-deterministic to a naïve observer.

That's just a very simple example, in a more complex system there could be all kinds of uncommon behaviours that are less simple to trace. So when modeling the system, they just box it as "there be dragons" and move on.