There is also the fact that it might realise that if it plays move A and the other player doesn't play the exact perfect move it's more winning and if it does it's repeated therefore playing move A first before trying the next best option of B is a better play.
This is in line with my thinking that it's "probing for weaknesses". AZ plays Rf6, analysing it as the best move because SF's only sound response is repetition, which AZ can decline. So it "tests" SF to see if it can refute Rf6, and when AZ observes that indeed SF can refute that move, AZ chooses a different move which maintains its edge. This is a human thing to do... we're hoping for the quicker win thanks to the limits of our opponent. Why is AZ looking for the quicker win? Why does it value a quick win over a long win?

I appreciate I'm just guessing as to its motive for playing a different move. It's just it raises interesting questions that I kinda like thinking about.