You did not do that right. You assumed that because it was not present one week that it has a greater chance the next. The chance of it being available is equal from 1 week to the next, it does not increase. The chance of "Plan C" appearing again this week is 1/14, the chance of it appearing in 2 weeks, 1/14. The chance of it appearing 4 weeks from now, 1/14. Just because it doesn't appear doesn't mean it's going to have a greater chance to appear. Players of Diablo 3 understood this when dealing with the Ancient Machine in Act 2. 10 outcomes of it, achievement required you to see all 10. It took me 1.5 years to get all 10 outcomes (played from launch), some people even longer, some never got it. With Xur, we're dealing with an even smaller chance, and to not see Gjall by next December wouldn't be all that surprising. Edit: They did say his chances of certain weapons is the same as loot drops, which means that if Gjall is weighted in the drops then it is also weighted with Xur. It's still RNG, chances of it appearing in the list doesn't change from week to week....unless you add or remove weapons from the list all together.