LLMs are mathematical functions executed on floating-point calculations. The software and hardware implementations cause non-deterministic side-effects.
In most cases, these non-deterministic side-effects are desirable to produce outcomes that converge towards better overall performance.
That being said removing the non-deterministic behavior is some cases can be incredibly difficult since this behavior may be buried in some framework. For example, weight initialization is performed by selecting random small values for regularization in neural network implementations. This is a desirable effect to prevent vanishing or exploding gradients, but that is not to say that those random values couldn't be explicitly reused in some use case if it was absolutely necessary.
When people talk about using CPU, vs GPU, it typically references computational speed increases but GPUs include a myriad of computational optimizations and some GPU optimizations for picking random numbers do exist and may be used. There are also floating point anomalies in some computation environments but that should not be attributed towards being "non-deterministic".