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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RNG does not mean that every item on a loot table has an equal chance of appearing. This goes all the way back to Dungeons & Dragons and the foundations of random treasure in roleplaying games. If you found a magic ring in a treasure chest or a merchants wares in a D&D game it was 15x more likely to be a Ring of Protection than a Ring of Three Wishes. The same principle applies to collectible card games or other game systems where relative power and rarity are one in the same. The reason why Obsidian Mind only appeared once and Sunbreakers appear all too often may simply be that the more powerful items in a loot table may only occur on a few number rolls (1-2 on a roll of 1 to 100) whereas more basic items can be tied to many more number instances (3--30 on a roll of 1-100)