KM I have found that for some screens, the 60 hz flicker comes from pixel inversion.
Here's a screen shot of my newly-arrived HP laptop using https://pixelinversion.com/ (disclaimer: I made this web site). When you locate the correct pixel inversion pattern you can clearly see the flickering with the naked eye and detect it using the Thorlabs photodetector on 100x zoom (actually, it's so bad you can see it at lower zooms as well but it's really hard to make out at 0x zoom).
So half the pixels are always flickering with the above pattern and the other half are flickering with the same pattern but offset by 16.6 ms (or something like that - my formula might be off by a bit). In theory, the two waveforms would cancel each other out, but because the waveforms are not perfectly symmetrical, the screen flickers.
Here's what the combined waveform actually looks like: