Howdy,
I would not worry so much about the unloaded Hz. I would tweak your settings to see what your average normal load is. After you see what your average load is, I would then adjust it to 60Hz. In other words, you have it powering your house. You have a few lights on and refrigerator running, saw your current draw is 3000 watts, showing about 75% on the gauge, set you Hz here to 60 and then totally unload you system and look at what you unloaded Hz is. The MEP-802A has a mechanical linkage. That is why you physically change the speed of the engine to set you Hz.
In the United States, electrical items are given a UL rating. Manufacturers make things to withstand a variable of 7-10% from normal to still operate. UL wants it to be a variance of 5%. All and all, things like to be at 240volts and 60Hz. That is also why it is wise to have a battery backup on big TV's and computers. Electronics really like to have what is known as clean power.