A calculator uses known deterministic algorithms to compute an answer. As long as you stay within the precision bounds, you'll get reliable and correct answers 100% of the time.
An AI is the result of randomness put through a simulated evolution, each one is different. We tend to expect them all to be the same, but that would be like redoing the big-bang until present, and expecting the dominant beings to be human, live in the same countries, speak the same languages, and believe all same cultural norms.
Nobody really groks how alien AI truly is.