Let me go down week by week, mmkay? DL:DR - Sunbreakers: 4x in 9 weeks, not since then /\ Voidfang Vestments: 7 times in 10 weeks, not since then /\ Starfire Protocol: 4 times in 3 weeks, all in the last 4 weeks. Proof that Xur is NOT RNG, and that we can expect Half Life 3 any day now.
1 - Sunbreakers
2 - Sunbreakers
3 - Apotheosis Veil
4 - Sunbreakers
5 - Voidvang Vestments
6 - Light Beyond Nemesis
7 - Voidfang Vestments
8 - Voidfang Vestments
9 - Sunbreakers
10 - Voidfang Vestments
11 - Voidfang Vestments
12 - Voidfang Vestments
13 - Light Beyond Nemesis
14 - Voidfang Vestments
15 - Claws of Ahamkara (wow something new!)
15 - Claws of Ahamkara (lol, jk)
17 - Starfire Protocol
18 - Starfire Protocol
19 - Obsidian mind (bricks were shyted)
20 - Starfire Protocol
Edit: Yes, Xur could be on a weighted system or a two roll system to determine which item he sells. I do not think so as his inventory is exactly the same as week 17. If Xur is RNG, then it is a really bad RNG.
Edit 2: The only other thing I can think of is that BUNGiE tracks which Exotics are in use and sells ones from a list that is compiled of all Exotics that are below a certain % of use in the playerbase.
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http://youtu.be/YLxHKQA-Eew deej says xur isent rng in this video not rick roll