As Sethos said, downsampling.
The Asus can display a true 2160p resolution which will look a hell of a lot better than downsampling. Also to those calling Asus out on only 30fps via HDMI, well the new 55/65" Sony 4k's only do 30fps via HDMI. Samsungs new 4k sets that come out soon will only do 30fps via HDMI. It is just a limitation of the HDMI port that causes this.
I just wish Sony/Samsung would put a Displayport plug on their sets and I would be there easily since DP 1.2 can do 60fps at 4k resolution. The Asus is going to be one of the first native 4k panels that will let you do 60fps because of the Display Port. But 4,000 for a 31.5 inch is outrageous.