It's likely because the SoC is different. This SD860 will consume higher power, other parts of the phone has to consume lower.
never00miss had an explanation understandable. [SD860 is SD855+ overclock or SD855 overclock overclock
]
https://xdaforums.com/t/poco-x3-pro-kernel.4273939/post-84997577
So they might have lower voltage for the screen brightness to preserve power for the chip. (for stability reasons)
As Gsmarena didn't detect the voltage supply to the screen and manually matched them between for devices, they are controlled by MIUI, Android and the "light sensor". (still stability reason)
Maybe there's way to remove the limit, especially rooted.
Unless they test it with root and had push it to the maximum they could, it's not yet the actual Maximum brightness. (eg. the thread about overclock, do the opposite, maybe you can get higher brightness at the cost of lower performance of the SoC,
at your own risk)
You can read some similar question and answers here
https://www.reddit.com/r/Android/comments/hbjd81
View attachment 5308881
This as well.
https://stackoverflow.com/questions/56203720/how-do-i-detect-the-screen-brightness-range-on-android