Look at the vertical line for 2,000 rpm in the Powerdynamo chart.
At 2,000 rpm with a ~7.5A load, you get 12.6V
At 2,000 rpm with a ~6A load, you get 14V
They don’t specify the open circuit voltage at 2,000 rpm (voltage with zero A load), but its likely over 20V.
Per the GT750 shop manual…
To test the alternator open circuit voltage, you bypass the voltage regulator & provide full battery output to the file coil (the rotor).
Then with the alternator output not connected to a load you get:
for 1,500 rpm the alternator output open circuit voltage is more than 16V
for 2,500 rpm the alternator output open circuit voltage is more than 27V
The GT-triples have a voltage regulator that controls the current going to file coil so that the alternator output voltage is correct.
The Powerdynamo uses a permanent magnet alternator.
Permanent magnet alternators don’t have a field coil.
Traditionally they use a shunt-type voltage regulator that controls the voltage by adding additional load to the alternator.
The T250/T350/T500 also have permanent magnet alternators.
They have switched alternators with charging & lighting coils. The lighting coil is switched on only when the lights are on.
The switch provides most of the alternator output control.
Beyond that they mostly rely on the battery to make sure the voltage doesn’t get too high.
A costly example...
Some time ago I was working on my 71 T350.
Disconnected the battery & adjusted the points.
Then was interrupted & forgot to reconnect the battery.
Then kick started the bike. After a few moments the neutral light went out.
No battery ==> no voltage regulator. And I burned out the headlight, neutral light & taillight.