Pitting usually starts on the pitch line and then spreads to other areas. There are small geometrical differences even in the most accurate gears which means that some teeth (or some areas of some teeth) are slightly more heavily loaded than others and these will be the teeth which pit first.
If I understood your application correctly - on open girth gear and pinions - the geometrical differences can be very large and it is not unusual to see some teeth with no damage and others with pits.
Hardness is a direct factor in the resistance to pitting and if there were large differences between the hardness of the teeth in one area compared to another this could produce the same effect but my observations would indicate that this is less likely than geometric effects causing the pitting to occur in some areas before it appears in others.