Strange as it may sound, the voltage is not changing.
These pins are using PWM ( Pulse width modulation). This means that the switch is on for a short part of time and then off for part of the time.
Sort of like ------------ . The on lines should be solid like the off lines and they are connected into one line.
The longer the voltage is high, the brighter the light will appear to be. Actually, things are happening so fast that we cannot see them.
That sort of explains what is happening.
Now then, the book says something about our eyes perceiving the brightness. It says we have problems seeing the difference as the light gets brighter.
I hope this sort of explains it enough.
Ken