Youtube resolution options on the iPhone 6s Plus -- not as high as those available on Chrome OSX; why is that?

Hey iPhone community -- I know this is a common problem in general: people uploading videos to Youtube and not having the resolution be as high as they would like. The "just wait" solution seems to work by and large, however, in this case of mine, I uploaded a video over a day ago and it is playable at 720, 1080, and even 2160 from my dashboard accessed via Chrome OSX. But on my iPhone 6s Plus, the highest I see is 720.

I can play other Youtube iOS videos at 1080, so I know it is possible on the 6s Plus, but any idea why my own video (which is processed as high as 2160 on Chrome OSX) is only 720 on Youtube iOS?

Thanks everyone.