Your calculations are right, but what you are missing is that you have arbitrarily selected particular even (4x voidfang in a row) as "strange" when there is no evidence of that. If I flip 10 coins and I get the sequence HTTHTHTTHH, it looks normal and completely unsurprising. However, there is less than a .1% chance of getting that sequence out of ten flips. In fact, it is the same as the sequence TTTTTTTTTT, which you would probably say is very unlikely. No matter what sequence of exotics you consider, the probability of Xur having those particular exotics is always very, very low. But he has to have SOMETHING, and so one sequence will definitely happen. The trick is that there are a huge number of sequences, all of which occur with low probability. So afterwards you say, that can't be RNG because the chance of this happening is so low. But it could have been 2x voidfang and 2x sunbreaker, of 4x sun breaker, or any other combination that you want. There are so many unlikely events that the probability of at least one of them happening becomes quite high. For instance, the probability that at least one exotic armor appears four times in a row on Xur over a period of 3 months is over 15%. It just so happened to be Voidfang, but there was no reason it couldn't have been another.