I'm no expert on how it works, but it's called packet sniffing. With the right software you can read all the information sent from destiny servers to your console. With that information it is possible to data-mine future information. The first person to do it was a reddit poster from Japan who goes by megaman. Since then a few others have started doing it as well. He's been correctly predicting nightfalls, weeklys, zur inventories, etc. for over a month, but it's only been common knowledge for a couple weeks as more people realize he's right. He's also predicting future iron banner dates and weapons sold. Bungie will probably change things tho now because it's become common knowledge, but I don't think that will stop someone from just doing it again. It doesn't mean, however, that zur is not rng. He may not be but it doesn't prove anything. It simply means that if it is rng, rng picks zur's inventory well in advance. I personally think knowing the future is taking the fun out of zur day and I wish they would stop.

Some losers with way too much time on their hands supposedly data mined some game files that were included with the 1.1.1 update. The files showed Xur's inventory and the Nightfall for a couple weeks. This just proves that these things are rolled several weeks out, which actually makes sense given their update schedule.