Believe it or not, it's purely random. Yeah, it scored some unlikely 21 combos on me too, several times during testing ^^
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Ah, the restaurant guy. There's no difference between him and Johan in the lounge except for the bet amount. I just checked his logic. So, getting several wins with a perfect 21 is either really bad luck or some changes to the game. By statistics, it's highly unlikely to happen in a prolonged time period.