the voltage is...REGULATED
you have to remember one thing...the alternator only exists to "top off" the battery which is actually powering everything.
if the battery voltage goes down the alternator output goes up to compensate and keep it "level"...
revving the engine to 2000 doesn't really change the electrical power demand of the car over 1500rpm or 3000rpm.
the only time you might expect to see a change in output is below the "minimum rpm range" of the alternator which is probably about engine idle speed.