You can justify post facto the move you made. Can you write down the logical process (probably a combination of strategy, move generation, analysis and selection) that would replicate the moves you picked from the beginning of the game? I don’t even need the brain chemistry (the implementation), but I don’t think even writing down the logic that generates those moves (in contrast with a logic to justify each move after it was generated) is something humans can do in most cases.

Are they actually the reasons for your move, or are they post-hoc justifications, and how would you know?

You are not explaining your internal processes (neurobiology, brain chemistry, neuron firing patterns etc.) that caused you to arrive at given moves. You will only provide proofs that your moves are good.