[Someone]

Are they? AFAIK, the “etc” includes using hardware that produces the same results for a given input every time. Once you start to multi-thread/multi-process in combination with floating point math, that can be hard to accomplish.

For example, the result of summing a stream of floats depends on the order the floats arrive in, and that order can change depending on what’s in your CPU cache when you start a computation, on whether something else running on your system such as a timer interrupt evicts something from cache during a computation, etc.

If you’re running on your GPU, even if the behavior of your GPU is 100% predictable (I wouldn’t know of that’s true on modern hardware, but my guess is it isn’t) anything that also uses the GPU can change things.