Why can't the "Time interval corresponding to the difficulty button" be modified?

I can’t upload the picture, so let me give you a few examples to show where the “time interval corresponding to the difficulty button” is.

Maximum number of reviews/day
9999
Time interval increase ratio for simple answer
150 %
Time interval for the Difficult button“It can’t be modified here. Why?”
120 %
时间间隔因子
100 %
最大时间间隔
10 天