So, when I play UHD material I will get 128 pixel black bands on the left and right edges if I have a "True 4K" display, and no letter-boxing if I have a UHD display.
Yawn.... I don't see that the difference is significant.
The LG 34UM95 has about 5M pixels, whereas a UHD monitor has 8M plus. The big difference is 1440 vs 2160 vertical pixels. LG is releasing a true 4K 31" a thunderbolt 2 monitor this summer with 4096x2160 pixels with model number 31MU95. If you are working with 4K or UHD video, then there is a big difference between those resolutions and the LG 34" ultrawide because it can not show all the pixels of the video.