How does the brightness of an LED depend on the average voltage?
The apparent brightness of a PWM driven LED to human visual system is a very logarithmic curve.
It means you can easily see a big difference between PWM signals of 1%, 2% and 3%, but will not see hardly any difference at all between PWM signals of 97%, 98% and 99%.
The average brightness of the LED to a light meter will still be linear in respect to the PWM duty cycle.