Yeah... the PWM is perfectly linear... It's our perception that's approximately logarithmic. So, a PWM change from 10-20 (a 100% increase) will appear to be a bigger change in brightness than a change from 100 to 110 (a 10% increase).
If the change is slow enough, say sunrise effect over a 10 minute period, you can't really see the change happening anyway, so you won't notice any "nonlinearity" in the rate-of-change.