For the benefit of anyone who is interested in what is going on, who doesn't have an Arduino Uno R4 and/or an oscilloscope, here is a video showing how the pulse width varies using the IDE example 'Sweep'.
The IDE Example 'Sweep' has been modified by changing the delay(15) to delay(100).
The pulses are repeated every 20ms.
The width of the pulse can be seen to jump in steps of 100µs.
Compare that with exactly the same code on an Arduino Uno R3:
The change in pulse width is far smoother.