Some screens use DC dimming. It isn't an issue for most people, but post-concussion I've had to be pickier about screens. The HP gave me a headache with the 200Hz PWM. The Legion lets me use brightness levels other than 100% without a headache!
Yes, it's about the frequency and filtering. Some screens use ridiculously low switching frequencies that mean the resulting signal can't be filtered out with anything that would fit in a thin laptop.
Isn't that the normal way to dim an LED? Was the frequency too low, or what was the issue?