all down to "nature's" clock
Flat-spot, I believe the evening timing is very much dependant upon the timing of sunset, which used to be marked with the firing of a field-gun.
The actual start of the Ramadan period is marked with the sighting of the New Moon. The first person to see this and report it, used to get a reward. In recent times, the rules were "bent" on several occasions by sending up someone in an aircraft, to high altitude, so that he would be the first person who could see the moon breaking the horizon!