3840 keeps the aspect ratio 16:9. 1920 is not 2k. 2048/2000 would be 2k, 4096/4000 would be 4k. That is why it is called UHD and not 4k when the resolution is 3840x2160. In the film world, it was often referred to as having 2k scans but being delivered HD instead since it was most often scanned to video tape. Not everyone was able to deliver digital files (DPX sequences) at true 2k, and less were able to accept them. So 2k just became interchangeable as HD deliverables in the film/video realm.