What causes Duty Cycle to decrease from fluctuating 50% at idle to fluctuating 27% at 2500 rpm?

I have a California 1990 300e with a 103 engine. The car has 121k miles, has got new cap, rotor, plugs, wires, OVP, injectors, fuel pump, accumulator, sensors on the head and filters.

The engine starts, idles and runs well. My concern is low gas mileage -17.5 MPG mixed city and highway driving.

Duty Cycle Measurement @ Operating Temperature:
Key on: 50%
Idling: inititally fixed 50% for @30 seconds then fluctuating +/- 3%
2500 RPM fluctuating 27%