Replay is not recorded fight. It is "re-arranged" fight.
When you attack, they store only when and what have you deployed, and when you push replay, what game does, is deploy units in same order and positions as you did, but the fight is whole new. Sometimes, this can lead into different results.
That is also a reason why sometimes are replays unavailable. If they change any game mechanics (like dmg output, or paths or whatever else relating to combat), the replay would not reproduce the same result and is unavailable.
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