Depends if its a synchronous or inverter generator, a inverter gen does control engine speed to govern o/p voltage, a synchronous one does not, as implied the speed of the engine/gen sets the o/p frequency.
The holes in the arm are probably to setup the gen from new, there will be slight variances from gen to gen so the manufacts will select the best position for that set.
Or if you were to run the set from lpg, or diffrent grade fuel then you could use a diffrent position to compensate, a gen set run on lpg can hunt, as lpg is a gas and has a diffrent calorie rating when it burns, upsetting the performance of the governor making it hunt.