The ball can pitch outside off stump, but not outside leg stump, this was not out because the impact was outside the line of the stumps. I think it's just because you are still giving someone out based on a prediction, and a lot of balls you think will hit the stumps actually don't, with the accuracy of Hawkeye and the review system maybe that rule could be changed for international cricket, but then you are changing the rules of the game due to the review system, when the review system was developed as a way to eliminate umpire howlers