Given the same input, a compiler will always produce the same output, down to the last bit. The same cannot be said about AI

LLM's have something like a temperature setting that is producing the intentional randomness, so that you prompting it is not going around in circles all the time. (if you are running an army of agents this is actually important for usefulness)

Anyway this is not an inherent LLM thing, it can be turned off.