The odds decrease, yes. Probability tells us that much. But you can't use probability to determine if randomness exists, so your original point is moot. Hell, unless people can get their hands on destiny's source code we don't actually know if Xur is random at all. He could just be following a really, really long pattern. We can look at the odds of this particular pattern occurring and notice that they're astronomically low, but we still can't know for sure. http://www.random.org/analysis/dilbert.jpg