You need to understand that the battery controls the voltage largely. A low battery will suck a lot of current at 12.6 volts. As its state of charge rises, you can get it up to 13 or 14 volts with the same current.
The alternator produces voltage in proportion to its speed when off load and it has to reach a speed high enough to start to charge the battery. Above this speed, the voltage is not really regulated, it just rises unless enough curent is drawn to hold it down.
the actual system voltage is a balance between the two devices. The alternator producing current and the battery aborbing it. Roughly speaking the wind determines the current and the battery state of charge determines the voltage.
As the battery gets full it needs no more charge current, and its voltage rises even with a small current. Above 14 or 15 volts you begin to shorten the battery's life. A brief charge at 15 volts is good if the battery has been low and needs waking up. But prolonged charge at 15 volts will wear it out.