The data is full with invalid results.
All 50Hz and 100Hz are the results of your ambient lights with 50Hz or 100Hz AC electricity.
All results less than the devices' refresh rates are also invalid. You can't have PWM frequency less than refresh rates. If so, there will always be a couple of frames that are never lit by backlight. (Assuming the pulse width is very short at low brightness)
On OLED device, the PWM frequency must be the multiple of refresh rate, that's how OLED works. So results like 250Hz on 60Hz device are all invalid. 240Hz should be the actual result.
there are some results that are clearly just measuring noise.
Last but not least, there are some results that are simply typos. For example
Nokia-T20-Tablet-review-Pad-with-LTE-lasts-forever
Aside from invalid results, the numbers shown here didn't take the modulation depth into consideration. For example, a 50Hz PWM with 100% modulation depth is easily noticeable, but a 10Hz PWM with 1% modulation depth is barely perceivable. If you check the waveform, some of the wave pattern is very far from the time axis, indicating the modulation depth is very small.