I am not surprised. They are using RED cameras and those are capable of 5K and 6K. Why not use the highest resolution possible? Makes sense!
Last year's "House of Cards" was shot in 5K. I guess they have upgraded their cameras this year.
Even if they are not planning to offer 6K streaming anytime soon, it helps to have a higher resolution master. You can always downrez it to whatever resolution you need.
Many of movies released in HD were originally scanned at 4K from the film.
There is also a special issue with 4K: professional (theatrical) 4K resolution (4096 x 2160) is not the same as the TV 4K (3840 x 2160) and converting from one to the other presents some problems. It is much better and easier to just oversample and master everything at 5K or 6K and then to downrez it to whatever resolution you need. The main additional cost is the extra disk storage required. But disks are getting cheaper and cheaper every year.
