4K resolution
As noted above, DCI 4K is 40962160, while 4K UHD is 38402160, producing a slight difference in aspect ratio rather than a significant difference in resolution. Traditional displays, such as LCD or OLED, are 3840 pixels across the screen, with each pixel being 1/3840th of the screen width. They do not overlap—if they did, they would suffer reduced detail. The diameter of each pixel is basically 1/3840th of the screen width or 1/2160th of the screen height - either gives the same size pixel. That 38402160 works out to 8.3 megapixels, the official resolution of 4K UHD (and therefore Blu-ray UHD discs).
5K resolution
In comparison to 4K UHD (38402160), the 16:9 5K resolution of 51202880 offers 1280 extra columns and 720 extra lines of display area, an increase of 33.% in each dimension. This additional display area can allow 4K content to be displayed at native resolution without filling the entire screen, which means that additional software such as video editing suite toolbars will be available without having to downscale the content previews.