Regular alternators have to keep running at a constant speed to maintain a constant power output voltage and frequency.
Thus, even if it's only powering a 10W bulb, it's still running at that high speed.
An inverter generator delivers a constant output frequency and voltage independent of the motor speed (within limits), so the motor can slow down for light loads and only has to run at the maximum speed when fully loaded.
Thus, noise and gas consumption are significantly reduced for light loads, compared to a standard AC alternator.