The short version is the main shooter on the 2 cheaper devices is a 12mp camera. 8k video 'requires' at least 33mp (or something liek that), so when you shoot in 8k, you are using the 48mp telephoto camera, which crops in and doesn't have as good a sensor. The Ultra has that monster 108mp main camera so when you shoot in 8k you are actually using the main (best) camera. MKBHD explains it WAY better in this vid, or maybe the S20 one, but that is how I understand it.