Consult for the rationale for HDR/SDR balance setting

In the Windows HD Color settings

There is an option for HDR/SDR brightness balance

I am using a monitor with 1200nit peak brightness and when I tried to change the setting I noticed that 

① When the luminance balance slider is dragged to the far left, the signal V of SDR white content is close to 0.5, and the actual luminance after electro-optical conversion is about 100nit, which is in agreement with the theory of SMPTE2084 curve.

② When the luminance balance slider is dragged to the far right, the signal V of the SDR white content is close to 0.666, and the actual luminance after electro-optical conversion is about 500nit as indicated in SMPTE2084 curve. I want to ask the reason choosing this value as the maximum brightness in windows.