Alright, thanks for clearing that up. So basically, this could be solved by software calibration, which Apple somehow doesn't prioritize in the foreseeable future.
Anyway, I think mine's fine. The difference based on your test isn't big in my case, and it may partly be caused by the full-front glass screen protector that's on it, which also covers the notch (which I like better than those screen protectors that don't cover the notch).
Anyway OFF-topic here, but important: After little more than a week of usage, I now mostly dimm my screen through Reduce-whitepoint accessibility shortcut. That's because the normal brightness reduction does cause slight headache after longer use, caused by OLED PWM control. In my opinion this is a major problem. Using reduce-whitepoint method seems to prevent the flickering partly. Let's hope apple's next generation panels don't use PWM anymore.
So far these are my ONLY complaints with the 12 (mini), in order:
1. PWM flickering of the display causing headaches with longer use on low brightness.
2. Lens flares in the sun and internal reflections at night with the main camera. The iPhone 11 and 12 have this problem MUCH more than iPhone 8 and earlier.
3. Mandatory swipe-up to unlock after FaceID is inconvenient to do one-handed.
Other than these, the phone is pretty much perfect...