Lauda89 When you took the measurements with your Opple LightMaster, did you test the same color for both devices? I'm guessing you tested a brighter color (like white) for the Macbook and a dimmer color (like gray) for the gaming PC.
It would be best to use the same colors for both. And also to try using a handful of different colors, comparing the same color on one device with the same color on the other device.
Here's a handy page that lists all 256 shades of gray, red, green and blue. Obviously you don't need to test all 1000+ colors, but testing 20 different colors might produce interesting results. https://pixelinversion.com/AllColors.html
Also, it would be interesting to see the waveform from your iPhone X. 99.5% modulation depth is pretty crazy for a screen.