Framerate vs Brightness

I was looking for the minimum framerate of my project to set a safe limit value (led-limit-refresh) and avoid flickering as much as possible

To my surprise, I noticed that the speed is affected by the brightness in a random way, it’s not a real problem but I found it interesting, maybe it’s a specific problem of mine?

Bright   Average
50%       395hz
52%       404hz
59%       433hz
65%       404hz
69%       404hz
70%       375hz
80%       440hz
85%       416hz
90%       440hz  
96%       410hz
100%      366hz