Yes, several of us have observed that phenomenon. I dubbed it the
Carrot Effect.
The paradoxical result: I typically get better workouts in SPORT than in ECO or OFF. And it's not just a feeling. I can see the Carrot Effect in action on my rider power display.
Bottom line: On a well-designed fitness bike like my Vado SL 1, you can layer as much exertion as you like onto any assist level.