Remember seeing once, i think it was an ex-ref, saying that most refs dont actually stop their watch whenever the ball goes out of play, they just guesstimate as the game goes on and then at the end, they decide how much time has amassed throughout the game and put in a 'rough estimate' as the alloted period of added on time. The 'at their discretion' thing gives them a get-out now matter how wrong they get it. Sounds seriously shoddy and random, to me.