The alternator is an AC generator. The raw AC voltage varies with speed of rotation, but the elctronics that convert the AC to DC also control the voltage output. Thus an alternator in good order will produce an output voltage that remians more or less constant. The speed at which this starts to drop below spec is very low, which is one of the reasons why we ditched dynamos (DC generators).
I think you are confusing voltage (electromotive force) with amperage (current). Voltage is equivalent to pressure in the system, amperage is the flow. The current into/out of the battery is always a net figure, current demand from the electrical system less the current supplied by the alternator. Usually this is a flow into the battery which keeps charging during normal use. When you operate the starter, this turns to a large discharge. If you overload the system, e.g. with too many accessory lamps, you can produce a state of discharge in normal running. You get a flat battery when this happens.
Also, if the drive belt is loose, you can get a good voltage at idle and off load, but when you load up the electrical system this causes the alternator to start to stall, the belt to slip and to loose current.
Not sure if this helps, but I've done my best!