People are rounding down to the common resolutions used for media (480, 720, 1080, etc.). Since the XR doesn't have 1080, the next standard-size resolution is 720. In fact the XR does have an 828 display.
It's a matter of preference. Some people want a point-for-point match with their media and the screen, others don't care so much about it.
Thanks so much for the info, i guess 828p is okay, i am using iphone 8 now which has a resolution 1792-by-750- resolution and i dont see pixels. I was watching a XR reveiw on youtube and saw pixelation of XR is visible, i am sure its because of the camera which they use to record the video on and they wont be visible to our eyes.