This happens when the accumulator overflows.
It is explained in the description above the figure you are referring to.
the description above the figure explains the duty cycle add one clock when the accumulator overflows but not the pwm period. I think the pwm period add one clock when it is period dithering.
You are right, the period should not be changed with PWM Edge Dithering.
Let me report that, thank you for pointing it out.