The reason why 90hz is supported at 75% + brightness.

[anon(9918034)]

Its not the battery size either, here is the link to the google git.

https://android.googlesource.com/pl...c841b029311400ef2602bafe9b3776310d5e8a7^!/#F0

Edit: For those don't want to read through the page

The display uses different gamma curves for different refresh rates. It's hard for panel
+ vendor to tune the curves to have exact same brightness for different refresh rate. So
+ flicker could be observed at switch time. The issue is worse at the gamma lower end.
+ In addition, human eyes are more sensitive to the flicker at darker environment.
+ To prevent flicker, we only support higher refresh rates if the display brightness is above
+ a threshold. And the darker environment could have higher threshold.

 

[bhatech]

Good thing I can't make difference either way so not affecting me one bit

 

[anon(9918034)]

Good thing I can't make difference either way so not affecting me one bit

According to this the display will automatically switch between 90 and 60. At 75% brightness and higher the switch isn't noticable. However below 75% there is a screen flicker. To avoid a whole bunch of people complaining about that. They chose to limit the brightness to 75% and up to give the user a seamless experience.

 

[Beachboy63]

Good thing I can't make difference either way so not affecting me one bit

I'm in the same boat. I was wondering if I was the only one who couldn't tell the difference between the 60hz and 90hz refresh rates. They look exactly the same to me. My old eyes, I suppose.