I don't know what it IS doing.
But what it SHOULD be doing is pulling current from the battery when the battery is overvoltage.
And it should pull MORE current than the mill can produce at the windspeed where its output is maximum. Otherwise a wind near the magic speed will overcharge your batteries. (When the battery gets back down to or below the setpoint, the controller can turn the load off again.)
Think of the battery as a reservoir, the genny as a river running into it, the dump load as a spillway running out of it, somewhere below the top of the dam, and the voltage as the water level. You don't care WHICH molecules of water go in and which go out, or whether the spillway is near the river or on the opposite side of the reservoir. Once the dam reaches rated capacity you just want the spillway to dump water AT LEAST as fast as the river can deliver it, to keep the dam from breaking.
Of course "at least as fast" means "faster", since you can't hit it dead on and "slower" means "dam breaks".