RNG doesn't necessarily mean every item has a fair chance of dropping. Some items will drop more. Also, I don't know if you've ever looked at an RNG system for an MMORPG style game, but some legendary items drop more often than others. I haven't seen Bungie's RNG but I'm sure they have some items rarer than others and god knows what other system they have up for Xur. Just be glad he hasn't sold the same weapon for 6 weeks in a row. I'm glad I've gotten a new weapon every week for the last 4 weeks.